H2S Scavenging Capacity and Rheological Properties of Water-Based Drilling Muds.

Drilling hydrocarbon formations where hydrogen sulfide (H2S) is present could lead to the carryover of H2S with the drilling mud (i.e., drilling fluid) to the surface, exposing working personnel to this lethal gas. Additionally, H2S is very corrosive, causing severe corrosion of metal parts of the drilling equipment, which in turn results in serious operational problems. The addition of an effective H2S scavenger(s) in the drilling mud formulations will overcome these health, safety, and operational issues. In this work, zinc oxide (ZnO), which is a common H2S scavenger, has been incorporated into water-based drilling mud. The H2S scavenging performance of this ZnO-containing drilling mud has been assessed. Additionally, drilling mud formulations containing either copper nitrate (Cu(NO3)2·3H2O) or potassium permanganate (KMnO4) have been prepared, and their H2S scavenging performances have been studied and compared to that of the ZnO-containing drilling mud. It has been observed that the scavenging performance (in terms of the H2S amounts scavenged up to the breakthrough time and at the saturation condition) of the ZnO-containing drilling mud is very poor compared to those of the copper nitrate-containing and KMnO4-containing drilling muds. For instance, the amounts of H2S scavenged up to the breakthrough time by ZnO-containing, copper nitrate-containing, and KMnO4-containing drilling muds were 5.5, 15.8, and 125.3 mg/g, respectively. Furthermore, the amounts of H2S scavenged at the saturation condition by these drilling muds were, respectively, 35.1, 146.8, and 307.5 mg/g, demonstrating the superiority of the KMnO4-containing drilling mud. Besides its attractive H2S scavenging performance, the KMnO4-containing drilling mud possessed more favorable rheological properties [i.e., plastic viscosity (PV), yield point (YP), carrying capacity of the drill cuttings, and gelling characteristics] relative to the base and the ZnO-containing and copper nitrate-containing drilling muds. The addition of KMnO4 to the base drilling mud increased its apparent viscosity, PV, and YP by 20, 33, and 10%, respectively. Additionally, all tested drilling muds possessed acceptable fluid loss characteristics. To the best of our knowledge, there are so far no published studies concurrently tackling the H2S scavenging (i.e., breakthrough time, breakthrough capacity, saturation time, saturation capacity, and scavenger utilization) and the rheological properties of water-based drilling muds, as demonstrated in the current study, highlighting the novelty of this work.

Introduction

Drilling muds are an essential part of any drilling process, and they serve many important functions including cooling and lubrication of the rotating bit and downhole equipment, transport of drill cuttings to the well surface, and exerting pressure on the surrounding formation to prevent an undesirable influx of fluids or caving in of the borehole.[1−5] To achieve such objectives, drilling muds must possess special rheological properties, which are frequently achieved through the use of different mud additives.[6−12] For instance, for effective transport of drill cuttings, the drilling mud must display a shear thinning behavior.[13,14] Additionally, when circulation is stopped, the fluid should possess a sufficient viscosity to attain a gel-like structure that maintains the solids suspended and prevents their return to the bit area.[4,15] This unique rheological behavior can be obtained by a proper mud formulation using different additives; each one serves a certain purpose.[16−19]

A common scenario often encountered while drilling subterranean formations is the infiltration of drilling muds by pockets of hydrogen sulfide (H2S). H2S is a toxic, flammable, and colorless acidic gas with a rotten egg smell at low concentrations.[20] It is a heavy gas with a specific gravity of 1.18 compared to air. This makes it sink to the lowest possible level and escape to the confined spaces.[21−23] H2S can form explosive mixtures with air in a wide concentration range of 4.3–45% with an ignition temperature of 260 °C.[24] It has been reported that the number of sour wells is increasing worldwide, especially for old oil fields, where the existence of H2S was not previously witnessed.[25]

H2S is considered to be one of the major health and safety issues in the oil and gas industry, not only for being dangerous to the working personnel but also for damaging metallic structures. It is a very lethal gas and extremely toxic to the working crew at the rig site. If not timely controlled, it can result in injuries, fatalities, fire, and explosion. It causes corrosion problems to all the upstream, midstream, and downstream steel structures. Pitting corrosion, hydrogen embrittlement, and sulfide stress cracking are all potential failure causes for drilling assembly, pipelines, processing units, and storage tanks when the handled streams contain H2S. For example, cracking failure of high-strength steels can occur within minutes in the presence of only 50 ppm H2S.[26,27] Moreover, the presence of as low as 0.1 ppm H2S in the handled streams can significantly reduce the equipment lifetime.[28] Accordingly, the presence of H2S carries significant safety, health, and economic penalties, requiring the effective removal of this lethal gas during drilling operations. This objective might be achieved through the addition of appropriate H2S scavengers to drilling muds.

H2S scavengers can be either in a solid or liquid form. The solid form can be soluble or insoluble in aqueous solutions (i.e., water-based drilling muds). Different types of H2S scavengers exist in the gas and oil industry nowadays. These scavengers are mostly based on triazines, ethers, aldehydes, amines, and metal-based compounds.[29−33] Among these H2S scavengers, metal-based compounds are very attractive because of their affordable cost, effectiveness, and safety factors. Metal-based scavengers react with the dissolved H2S in the drilling mud, forming insoluble metal sulfides. The metal sulfides are thermally stable and will not regenerate H2S under drilling operating conditions, thus providing a safe way for sequestering H2S.[2,28,34−37] Most of the commonly used metal-based H2S scavengers in the oil and gas industry are based on copper-, zinc-, or iron-containing compounds. Copper compounds such as copper carbonate (CuCO3)[38−40] and basic copper carbonate (Cu2(OH)2CO3)[30,36] were among the earliest metallic scavengers to be successfully applied in drilling processes. Reactions of copper compounds with H2S are typically fast and efficient and lead to the formation of solid CuS or Cu2S, both of which are highly stable and insoluble in acidic or basic media.[36,41] However, copper compounds suffer from one inherent drawback, that is, their tendency to accelerate equipment corrosion by spontaneous copper plating.[2,29,30,32,38−40] This type of corrosion occurs because copper, being a more noble metal than iron, tends to deposit on steel surfaces and creates a local corrosion cell that promotes the corrosion of iron on the surface. Consequently, copper-based scavengers are no longer used in the industry and have been largely replaced by scavengers based on zinc or iron compounds, which do not suffer from corrosion tendencies.

Currently, zinc compounds are by far the most widely used scavengers. They emerged in the 1970s as a substitute to the problematic copper-based scavengers that offered a similar degree of protection against sulfide species without the additional corrosion damage.[30,41,42] Zinc compounds are amphoteric; that is, they can act as acids or bases depending on the surrounding conditions. Consequently, many solid zinc-based scavengers would dissolve at highly basic conditions (water-based drilling muds are basic) to yield free zinc ions capable of scavenging H2S.[29,31,39,43] However, at pH greater than ~ 11, zincate ion Zn(OH)42– could form. While the zincate ion might still be effective in scavenging H2S, excess amounts of Zn(OH)42– could negatively interfere with the mud rheological properties, which might be detrimental to the drilling mud performance.[32,33,44,45]

An alternative option to copper and zinc compounds are iron compounds. However, unlike zinc and copper, where metal sulfides form through a simple precipitation reaction, the interaction between iron compounds and H2S is complex and might involve multiple steps and reaction products.[41,46] Generally, six iron sulfide minerals could potentially form as a result of the scavenging reactions (see ref (47), for example); the reaction rate and type of product(s) formed are largely dependent on the ambient conditions including the oxidation state of iron in the parent oxide (Fe2+ or Fe3+), temperature, pH, and reaction time, among others.[41] Currently, the only commercially available iron-based scavenger is the specially prepared, porous magnetite (Fe3O4), which is marketed under the trade name “Ironite Sponge”.[30,31,39,48] This is highly insoluble iron oxide that reacts with H2S to form the very stable pyrite.[27,40,49,50] However, being an insoluble solid, the reaction between magnetite and H2S is mostly limited to the external particle surface, which quickly becomes coated by a layer of reaction products that inhibit further reaction inside the particle, leading to an incomplete conversion (i.e., utilization) of magnetite.[32,41] Additionally, because magnetite reacts primarily with H2S, which is only dominant under acidic conditions (HS– and S2– species are dominant under alkaline conditions),[41] the alkaline conditions encountered in drilling muds would result in a negative effect on the reaction kinetics.[35,37,46]

Another approach for removing H2S (or its dissociated forms) from drilling muds is through the use of oxidizing agents. Such chemicals, when applied in appropriate dosages, can oxidize H2S to harmless sulfur compounds such as elemental sulfur S0 or sulfate ions SO42–.[28,51] Examples of oxidizing agents include potassium permanganate (KMnO4), hydrogen peroxide (H2O2), potassium thiosulfate (K2S2O3), and calcium hypochlorite Ca(ClO)2.[28,29,38,51] Oxidizing agents typically react quickly and irreversibly with soluble sulfides and, owing to their high solubility, are easily dispersed in the drilling mud formulations.[28]

In this paper, we aim to study the H2S scavenging using water-based drilling mud formulations containing ZnO, copper nitrate, and KMnO4. In addition to revealing and comparing the H2S scavenging performances of these scavengers, their effects on the rheological and fluid loss properties of the base drilling mud will also be investigated. To the best of our knowledge, most of the published studies in this regard either tackled the rheological properties of drilling muds or the removal of H2S using solid-phase adsorbents. Thus, this study is a novel contribution to the knowledge in this field, filling some of the missing gaps by studying the H2S scavenging performance of commercially available materials and their impacts on the rheological properties of the base drilling mud. The only comparable studies, to the best of our knowledge, are those recently published by Elkatatny and co-workers[44,52] and Ghayedi and Khosravi.[1] However, Elkatatny et al.[44,52] focused only on the copper nitrate-containing drilling mud, while Ghayedi and Khosravi[1] utilized ZnO–graphene oxide composites to scavenge a small amount of H2S (∼800 ppm H2S) generated locally using sodium sulfide as the H2S precursor, unlike the case where H2S is supplied from an inexhaustible source as we implemented in this study.

Materials and Methods

The methodology adopted in conducting this study is depicted in Figure .

Figure 1

Research methodology.

Compositions of Drilling Muds

The water-based drilling mud (i.e., the base drilling mud) was prepared by adding various additives such as the defoamer, XC polymer, potassium hydroxide (KOH), starch, potassium chloride (KCl), and calcium carbonate (CaCO3) to distilled water. The compositions (i.e., formulations) of the drilling muds (i.e., the base and the H2S scavenger-containing drilling muds) used in this study are listed in Table . The drilling fluid additives were supplied by an oil field service company in Saudi Arabia. The concentration of additives was taken in weight percent (wt %). The ingredients of these drilling muds were thoroughly mixed in distilled water using a high-speed Hamilton beach mixer. The mixing sequence and time of mixing for each additive are given in Table . All the additives were mixed sequentially at a mixing rate of 21,000 rpm. First, the defoamer was added to water, followed by the XC polymer, KOH, starch, KCl, and CaCO3. At the end, the H2S scavenger (ZnO, Cu(NO3)2·3H2O, or KMnO4) was mixed in the base drilling mud for 10 min. Each additive was mixed at high shear for a particular time mentioned in Table . The pH of all drilling muds was maintained at 9–9.2 using KOH. The levels of the three utilized H2S scavengers (ZnO, Cu(NO3)2·3H2O, and KMnO4) were kept the same (i.e., 0.1 wt %). ZnO (99.9%), Cu(NO3)2·3H2O (99%), and KMnO4 (≥99.0%) were purchased from Sigma-Aldrich. The average particle size of ZnO is 6.58 μm.

Table 1

Compositions of the Base and H2S Scavenger-Containing Drilling Muds

additives	mixing time (min)	functions	conc. (wt %)	base mud	ZnO-based	KMnO4-based	Cu(NO3)2·3H2O-based
defoamer	1	antifoaming agent	8 drops	8 drops	8 drops	8 drops	8 drops
XC polymer	20	viscosifier	0.43%	0.43%	0.43%	0.43%	0.43%
KOH	5	pH controller	as required	as required	as required	as required	as required
starch	10	fluid loss controller	0.5%	0.5%	0.5%	0.5%	0.5%
KCl	5	shale stabilizer	3%	3%	3%	3%	3%
CaCO3	10	weighting agent	6%	6%	6%	6%	6%
ZnO	10	H2S scavengers	0.1%		0.1%
KMnO4	10		0.1%			0.1%
Cu(NO3)2·3H2O	10		0.1%				0.1%

H2S Scavenging

In each H2S scavenging experiment, 10 mL of one of the drilling muds is placed in a column. One end (gas inlet) of the column is connected to a gas cylinder containing 104 ppmv (parts per million by volume) H2S, 1046 ppmv CO2, and balanced N2. The other end (gas outlet) of the column is connected to an H2S detector with a minimum detection limit of 0.1 ppm. A demister is installed at the top of the column to capture moisture. A valve and a mass flowmeter are installed at the gas inlet (before the column) to control the gas flowrate. Additionally, a pressure gauge is installed at the gas inlet in order to measure the inlet pressure of the gas feed. At the beginning of each experiment, the valve is opened, and the gas flow rate is controlled at 100 mL/min. The sour gas is bubbled into the drilling mud in the column, and the H2S concentration in the outlet gas stream is monitored and recorded continuously until saturation is established (i.e., H2S concentration in the outlet gas stream reaches ∼104 ppm). All H2S scavenging experiments were conducted at room temperature (∼25 °C) and atmospheric pressure. The amount (in mg) of H2S scavenged per gram of the utilized scavenger up to the breakthrough time is calculated using the following equationWhere Q is the inlet gas flow rate (mL/min), ρH is H2S density (equivalent to 1.391 mg/mL at room temperature and atmospheric pressure), tb (in min) is the breakthrough time (i.e., when H2S concentration in the outlet gas reaches 0.5 ppmv), m is the scavenger mass in the drilling mud (g), f is a conversion factor (=106), and Cin and Cout are the concentrations (ppmv) of H2S in the inlet and outlet gas streams, respectively.

Similarly, the amount (in mg) of H2S scavenged per gram of the scavenger at saturation conditions (i.e., scavenging capacity) is calculated using the following equationwhere ts (in min) is the saturation time (when H2S concentration in the outlet gas reaches ∼104 ppmv).

Rheological Measurements

The rheological properties of the base and the scavenger-containing drilling muds were conducted according to the standard procedures of the American Petroleum Institute (API).[53] Each drilling mud formulation listed in Table above was agitated (using a Grace viscometer, model no. M3600) at various shear rates starting from 3 up to 600 rpm; the shear stress value at each applied shear rate was recorded. Plastic viscosity (PV), yield point (YP), and apparent viscosity (AV) were calculated using the Bingham plastic model by applying the following equations

In the above equations, Φ600rpm and Φ300rpm are the dial readings at 600 and 300 rpm, respectively.

The gel strengths at 10 s and 10 min of each drilling mud were measured by first shearing the mud at 3 rpm and then leaving it under static conditions for 10 s and 10 min before taking the measurements of the maximum deflection obtained at this low shear rate.

Fluid Loss Tests

Fluid loss tests were conducted in order to evaluate the fluid loss control potential of the base and the scavenger-containing drilling muds using the API standard procedure (API-13B). API recommends conducting the fluid loss test at 100 psi pressure and room temperature conditions for 30 min. A FANN Series 300 API filter press was used to perform the tests. In each test, 350 mL of the drilling mud was placed in the cup of the filtration cell that was loaded with Whatman filter paper no. 50 at the bottom of the cup. The cup was placed in the frame of the apparatus, and the cap was tightly closed. With the necessary connections in place, 100 psi pressure was applied using compressed air. The filtrate was collected in a graduated cylinder for 30 min at different time intervals.

Results and Discussion

H2S Scavenging Using the Base Drilling Mud

The performance of the formulated drilling muds in scavenging H2S was investigated by bubbling a sour gas containing 104 ppmv H2S into each mud. Figure shows the changes in the H2S concentration in the outflow gas stream as a function of time (i.e., breakthrough curves). The base drilling mud formulation (without H2S scavenger) showed a quite low scavenging capacity. The breakthrough time is less than 1 min, corresponding to less than 1.5 μg H2S scavenged per mL drilling mud at breakthrough. The total amount of H2S scavenged at saturation (when H2S concentration in the outlet gas stream reaches ∼104 ppmv) is estimated using eq to be around 24.2 μg/mL of the base drilling mud. This very low scavenging capacity of the base drilling mud is not surprising, given that it is merely based on H2S solubility in the drilling mud formulation. It must be noted that the base drilling mud, and also, all scavenger-containing muds, have no affinity for CO2, as judged from the unchanged inlet–outlet CO2 concentrations throughout the scavenging process.

Figure 2

H2S scavenging using the base and the scavenger-containing drilling muds. The flow rate of the inlet gas feed is 100 mL/min, and the concentration of H2S in the inlet gas is 104 ppmv. The scavenging experiments were conducted at room temperature and atmospheric pressure.

H2S Scavenging Using ZnO-Containing Drilling Mud

Figure also shows the H2S scavenging performance of the commonly used H2S scavenger (i.e., ZnO). Because all muds used in this study have pH values in the range of 9 −9.2, more than 98% of the dissolved H2S in the muds is expected to be in the HS– form.[41] However, the reaction between ZnO (and also other scavengers) and H2S might also take place at the gas bubble–liquid interface. Regardless of whether the scavenger reacts with H2S or with HS–, the stoichiometric ratio of the hydrogen sulfide/scavenger will remain the same. Consequently, all reaction equations will be written using H2S. Thus, ZnO will scavenge H2S according to the following reaction equation

According to the above stoichiometric equation, each gram of ZnO should scavenge about 419 mg H2S when ZnO is fully utilized. As shown in Figure , the H2S breakthrough time in the presence of 0.1 wt % ZnO in the base drilling mud is around 3.5 min, corresponding to an H2S scavenged capacity at the breakthrough of around 5.5 mg H2S/g ZnO, which suggests that only 1.3% ZnO is utilized up to the breakthrough time. The scavenged H2S amount at saturation (t = 95 min) reached around 35.1 mg H2S/g ZnO, which despite being around 6.4 times higher than the breakthrough capacity, is still quite low with only 8.8% ZnO utilization. Nonetheless, Dhage et al.[54] reported a lower H2S scavenging capacity (19 mg/g) using unsupported ZnO (from BASF). Another unsupported ZnO sample (from SudChemie) showed a H2S scavenging capacity of 32 mg/g,[54] which is slightly lower than the obtained value in this study. Li et al.[55] reported an H2S scavenging capacity (at breakthrough) in the range from 3.4 to 37.6 mg/g (corresponding to ZnO utilization in the range from 2.17 to 31.9%), depending on the ZnO loading and the support type, upon which ZnO was impregnated. A scavenging capacity of 8.9 mg/g (at breakthrough) has been obtained by Song et al.[56] using unsupported ZnO. It must be noted that the H2S scavenging performance of ZnO could be significantly improved through mixing it with other metals or dispersing it on an appropriate support with high surface area and porosity, as shown by a number of studies (see, e.g., ref[56−59] and the relevant references cited therein). However, such scavengers are still not commercially available, and their production costs are expected to be several folds higher than the commercially available (unsupported) ZnO. Thus, H2S scavenging using supported ZnO was not pursued in this study.

H2S Scavenging Using Copper Nitrate-Containing Drilling Mud

Figure also shows H2S scavenging using a drilling mud containing 0.1 wt % Cu(NO3)2·3H2O. The copper nitrate was dissolved in the drilling mud formulation without being calcined or supported on any support. The H2S breakthrough time and the H2S scavenged capacity at the breakthrough time in the presence of 0.1 wt % copper nitrate in the drilling mud are around 11 min and 15.8 mg/g, respectively. The H2S scavenged capacity increased to around 146.8 mg/g at saturation (t = 418 min), which is more than fourfold higher than the scavenging capacity of ZnO. The application of copper nitrate-containing drilling mud for H2S scavenging was studied by Elkatatny and co-workers.[44,52] They noted that copper nitrate was effective in eliminating H2S from a sour gas stream with a scavenging capacity that is almost three times higher than that of a commercial triazine-based scavenger. Elkatatny et al.[44,52] proposed the following reaction equation between copper nitrate and H2S

According to the above equimolar reaction equation, 1 g of Cu(NO3)2 would scavenge 181.8 mg H2S upon the full utilization of copper. However, when Cu(NO3)2·3H2O is used instead of Cu(NO3)2 (as it is the case in this study), 1 g of the scavenger would scavenge around 141.1 mg of H2S when copper is fully utilized. Accordingly, the scavenging capacity of 146.8 mg/g obtained in this study is slightly above the theoretical full utilization capacity of copper. The excess amount of H2S scavenged (5.7 mg/g) might stem from the reaction of H2S with the generated nitric acid.[60]

Boutillara et al.[61] prepared activated carbon using CuCl2, which was carbonized at 800 °C in the presence of N2 or CO2. The sample that was carbonized in the presence of N2 possessed the highest Cu loading of 25.43 wt %. This material showed an H2S scavenging capacity of 28.6 mg/g, corresponding to around 21% copper utilization. Huang et al.[62] impregnated activated carbon with solutions having different copper nitrate concentrations (0.05, 0.1, 0.15, and 0.2 M) and at different pH values (1, 2, and 3). After drying at 105 °C, the sample impregnated with 0.2 M copper nitrate at pH 3 provided the highest H2S scavenging capacity of 42.2 mg/g (excluding the contribution from the H2S adsorption on the activated carbon, which was 4.3 mg/g). Micoli et al.[63] modified zeolite with copper and zinc using ion exchange and impregnation methods. The highest H2S scavenging capacity (∼40 mg/g) was obtained for the sample modified with copper using the ion exchange method. For either method, copper samples showed as high as five times H2S scavenging capacity than zinc samples, in line with our observation herein. Wang et al.[64] prepared a series of 3D ordered mesoporous structures containing CuO and used them to scavenge H2S. The highest copper utilization was found to be 73.5% despite that an H2S scavenging capacity (147 mg/g) similar to the one obtained in this study was reported. However, the 147 mg/g reported by Wang et al.[64] also comprises the contribution from H2S adsorption on the CuO support. Accordingly, the H2S scavenging capacity of the mud formulation of 0.1 wt % copper nitrate reported in this work is higher than the previously reported values with the exception of Wang et al.[64] However, the material utilized by Wang et al.[64] is more expensive and not commercially available, unlike Cu(NO3)2·3H2O.

H2S Scavenging Using KMnO4-Containing Drilling Mud

In addition to studying H2S scavenging using drilling mud formulations containing ZnO and copper nitrate, the scavenging performance of a drilling mud formulation comprising 0.1 wt % potassium permanganate was also investigated, and the result is shown in Figure . As displayed in Figure , the use of this drilling mud formulation delayed the H2S breakthrough time to more than 90 min. This is much longer than the breakthrough times observed using other drilling mud formulations (see Table ), suggesting a fast and efficient reaction of KMnO4 with H2S. Besides the fast reaction between KMnO4 and H2S, this scavenger is also capable of scavenging a larger amount of H2S relative to ZnO and copper nitrate (see Table ). The amount of H2S scavenged (125.3 mg/g) up to the breakthrough time using the KMnO4-containing drilling mud is around 23 and 8 times higher than the corresponding values obtained using ZnO-containing and copper nitrate-containing drilling muds, respectively.

Table 2

Breakthrough Time, Breakthrough Capacity, Saturation Time, Saturation Capacity, and Scavenger Utilization

parameters	base mud	ZnO-containing mud	copper nitrate-containing mud	KMnO4-containing mud
breakthrough time (min)	0.5	3.5	11	90.5
saturation time (min)	39.5	96	418	550
breakthrough capacity (mg H2S/g scavenger)	1.4 μg H2S/mL base mud	5.5	15.8	125.3
saturation capacity (mg H2S/g scavenger)	24.2 μg H2S/mL base mud	35.1	146.8	307.5
scavenger utilization at saturation (%)		8.8	100	100

Additionally, the amount of H2S scavenged reached around 307.5 mg/g at the saturation condition. This value is more than 2- and 8.7-fold higher than the corresponding values obtained using copper nitrate-containing and ZnO-containing drilling muds, respectively, demonstrating the superiority of KMnO4-containing drilling mud. Under alkaline conditions (as it is the case in this study), KMnO4 reacts with H2S according to the following reaction equation[65]

According to the above reaction equation, 3 mol of H2S will fully consume 8 mol of KMnO4 if the above reaction proceeds to completion. In other words, under 100% utilization of KMnO4, 80.9 mg of H2S would be scavenged per gram of KMnO4. This value is much lower than the 307.5 mg/g mentioned above. However, the above reaction produces MnO2; we have observed that manganese oxides are good H2S scavengers (results not shown). Manganese oxides (MnO) react with H2S according to the following reaction equation

Other researchers have also reported H2S scavenging using manganese oxides. For example, Li et al.[66] reported that manganese oxides supported on alumina can scavenge around 120 mg/g (H2S scavenging was performed at a reaction temperature of 500 °C and above). Additionally, Huang et al.[67] showed that manganese oxides (generated via the calcination of manganese nitrate, supported on MCM-48, at 550 °C under air environment) are more effective than iron oxides and zinc oxide in scavenging H2S. Furthermore, Wang et al.[68] reported that composites of manganese/aluminum oxides are able to scavenge H2S with as high as 96% manganese utilization. Accordingly, the high H2S scavenging capacity of the KMnO4-containing drilling mud likely stems from the contribution of both KMnO4 and the in situ generated manganese oxides upon the reaction of KMnO4 with H2S. Considering the superiority of KMnO4, it might have a potential to be added to water-based drilling mud formulations when drilling wells in hydrocarbon-bearing subterranean formations, where H2S is present. It must be noted that despite being a strong oxidizing agent, a corrosion rate of metal casing in the presence of KMnO4 was much lower than that in the presence of copper nitrate (results not shown).

Rheological Studies

As shown in Figure and Table , KMnO4 is an excellent H2S scavenger, followed by copper nitrate, and then ZnO. However, in order for an H2S scavenger to be included in drilling mud formulations, it should not compromise the drilling mud rheological properties. Therefore, we have performed a number of rheological tests of the drilling muds in the presence of ZnO, copper nitrate, and KMnO4 and compared the obtained results with those of the base drilling mud. One of the performed rheological tests is the shear stress-shear rate up to a shear rate of 1000 s–1. Figure shows the shear stress–shear rate curves for the various drilling muds used in this study. As displayed in Figure , all drilling muds show pseudoplastic (shear thinning) behavior, which is a key requirement for effective transport of the drill cuttings.[13,14,69] Furthermore, the addition of ZnO and copper nitrate did not significantly alter the shearing properties of the base mud. However, the addition of KMnO4 resulted in a significant increase in shear stress, particularly at higher shear rates. This higher shear stress is associated with higher viscosity as will be discussed subsequently. Thus, pumping energy of the KMnO4-containing drilling mud would be higher than those of the other three drilling mud formulations. Nonetheless, such high shear stress of the KMnO4-containing drilling mud will improve the ability of the drilling mud in cleaning the well from the drill cuttings.[1]

Figure 3

Shear stress vs shear rate curves of the base and the scavenger-containing drilling muds.

A rheological property that can be extracted from the shear stress-shear rate curve is the AV. More precisely, the AV is the ratio of the shear stress to shear rate. Figure displays the AV values of the base and the scavenger-containing drilling muds. The base drilling mud has an AV of 14.9 cP, which remained almost the same (15.0 cP) upon the addition of 0.1 wt % ZnO or copper nitrate. On the other hand, the addition of 0.1 wt % KMnO4 increased the AV of the base drilling mud to 17.9 cP (about 20% increase). According to Fink,[70] an AV of ≥15 cP is required in order to achieve an API fluid loss of less than 50 mL/30 min. Accordingly, all scavenger-containing drilling muds satisfy this requirement. However, unlike the borderline AV values of the ZnO-containing and copper nitrate-containing drilling muds, the AV value of the KMnO4-containing drilling mud is around 20% higher than the minimum AV value required. Nonetheless, Perween et al.[71] stated that the recommended AV range for water-based drilling muds is 20–35 cP. Accordingly, the AV value of the KMnO4-containing drilling mud is the closest to the recommended range. A slight increase in the concentration of KMnO4 or the XC polymer (i.e., the viscosifier used in preparing the drilling muds in this study) in the base drilling mud will bring the AV value of the KMnO4-containing drilling mud to the desired range, unlike the case of the other drilling muds, where relatively higher amounts of the XC polymer would be required.

Figure 4

AV of the base and the scavenger-containing drilling muds.

Another important rheological property of drilling muds is PV. This property is related to the resistance of drilling muds to flow. Higher PV is associated with higher resistance to flow and vice versa. Accordingly, drilling muds with low PV are preferred in terms of pumping cost. On the other hand, the density of the drilling muds should be high enough in order for the drilling muds to exert sufficient hydrostatic pressure. Drilling mud density is usually manipulated through the addition of solids (e.g., weighting agents). However, the addition of solids usually increases PV of the drilling muds and, consequently, the mud circulating (pumping) cost. Accordingly, there is an optimum PV range of the drilling muds that has to be maintained. Figure shows the PV values of the base and the scavenger-containing drilling muds. As displayed in Figure , the PV of the base drilling mud is 7.3 cP. Upon the addition of copper nitrate to the base drilling mud, the PV remained unchanged. The addition of ZnO to the base drilling mud also did not significantly alter its PV (increased by only 2.7%). However, upon the addition of KMnO4 to the base drilling mud, its PV increased by around 33%, demonstrating the strong viscosifying characteristic of KMnO4. Perween et al.[71] and Ismail et al.[69] recommended that the PV value of water-based drilling muds should not exceed 25 cP. Accordingly, all drilling muds used in this study satisfy this requirement. However, considering the PV range (10 to 60 cP) for biodiesel-based drilling muds recommended by Li et al.,[72] only the PV value of the KMnO4-containing drilling mud falls close (9.7 cP) to this range. Increasing the concentration of KMnO4 and/or the XC polymer in the base drilling mud will bring the PV of the KMnO4-containing drilling mud to the desired range.

Figure 5

PV of the base and the scavenger-containing drilling muds.

YP is also one of the important properties of drilling muds. YP is defined as the minimum shear stress required to initiate fluid flow. It stems from the electrochemical attraction between the ingredients of drilling muds. YP is correlated to the drilling mud ability to lift the drill cuttings out of the wellbore; drilling muds with higher YP would have better lifting characteristics. However, higher YP values are associated with higher mud viscosities, increasing the circulation energy requirement. Thus, an optimum value that is high enough to ensure efficient lifting of the drill cuttings but at the same time is low enough to avoid excessive pump pressure is required. Such an optimum YP value for water-based drilling muds is below 50 lbf/100 ft2.[73] As shown in Figure , the addition of ZnO and copper nitrate resulted in negligible changes in the YP of the base drilling mud. However, the addition of KMnO4 increased the YP of the base drilling mud from 15.07 to 16.44 lbf/100 ft2 (about 10% increase). Such an increase in the YP of the base drilling mud upon the addition of KMnO4 suggests that some electrochemical forces have developed in the presence of KMnO4. The improvement in the YP (which is still significantly below the maximum limit) of the base drilling mud upon the addition of KMnO4 would improve the mud ability of lifting and cleaning the drill cuttings from the downhole. Consequently, the sticking tendency of the drill string and its torque would decrease,[74] resulting in a more efficient drilling process.

Figure 6

YPs of the base and the scavenger-containing drilling muds.

The carrying capacity (i.e., YP/PV) is another important rheological property of drilling muds. It is related to the ability of the drilling mud to suspend the drill cuttings and, consequently, its capacity to remove them from the wellbore.[75]Figure shows the carrying capacity (YP/PV) values of the base drilling mud and the scavenger-containing drilling muds. It has been reported in the literature that a YP/PV value of ≥0.75 is correlated with a good carrying capacity behavior of the drilling muds,[76,77] which in turn results in an improved wellbore cleaning performance. Accordingly, all three scavenger-containing drilling muds (in addition to the base drilling mud) possess a good wellbore cleaning and drill-cutting suspension ability. However, KMnO4-containing drilling mud displayed a YP/PV value that is around 20% lower than that of the base drilling, unlike the other two scavenger-containing drilling mud, which displayed comparable values to that of the base drilling mud. Although a threshold YP/PV value of 0.75 is required, higher YP/PV values will surge the annular frictional pressure loss and, thus, increase the equivalent circulating density in the wellbore, which may break the formation rock if the fracture pressure is exceeded. Accordingly, KMnO4-containing drilling mud has a better carrying capacity with less load on the circulating pumps, which are key requirements in drilling operations.

Figure 7

Carrying capacity (YP/PV) of the base and the scavenger-containing drilling muds.

We have also evaluated the effect of the scavenger addition on the gel strength of the drilling mud. The gel strength represents the ability of a drilling mud to keep the drill cuttings and the solid ingredients (e.g., weighting agent) of the drilling mud suspended when mud circulation is ceased. It originates from the particle interactions between the mud ingredients under static conditions. The gel strengths of the base and the scavenger-containing drilling muds measured at 10 s and 10 min are shown in Figure . The 10 min gel strength for all drilling muds is much higher than the 10 s gel strength, demonstrating the time effect on the gel strength of these muds. Figure also shows that the addition of all scavengers did not significantly alter the 10 s gel strength of the base mud. Furthermore, the addition of 0.1 wt % ZnO and copper nitrate had almost no effect on the 10 min gel strength of the base mud. However, the gel strength of the KMnO4-containing drilling mud increased by more than 3.5-fold upon keeping this mud under static conditions for 10 min instead of 10 s, demonstrating the superior gelling and suspension capacity of the KMnO4-containing drilling mud. Nonetheless, although high gel strength is required for efficient suspension of the drill cuttings under static conditions in order to avoid their settling and accumulation in the wellbore, causing drill string sticking; excessively high gel strength might lead to fluid loss, fracturing of the formation, and ineffective solid control.[78] High gel strength might also require a high torque when mud circulation is resumed. Accordingly, the gel strength should not be excessively high or low in order to avoid the above problems.[79] According to Katende et al.,[73] the gel strengths at 10 s and 10 min of water-based drilling muds should not exceed 15 and 35 lbf/100 ft2, respectively. Therefore, all drilling muds studied herein satisfy this requirement. However, drilling muds with higher gelling characteristics (below the maximum limits mentioned above) are more appropriate.[79] Thus, KMnO4-containing drilling mud seems more attractive. Nonetheless, further studies to explore any undesirable interactions of KMnO4 with the drilling mud ingredients are required.

Figure 8

10 s and 10 min gel strengths of the base and the scavenger-containing drilling muds.

Fluid Loss

Fluid loss tests were conducted in order to get insights into the fluid loss-controlling characteristics of the base drilling mud and how the addition of the H2S scavengers affects its characteristics. As displayed in Figure , the 30 min fluid loss from the base drilling mud is around 9.1 mL. The fluid loss from the ZnO-containing drilling mud is almost identical to that of the base drilling mud, indicating its negligible effect on the fluid loss-controlling characteristics of the base mud. Unlike ZnO, the addition of the other two scavengers (i.e., copper nitrate and KMnO4) increased the 30 min fluid loss to around 11 and 13 mL, respectively. It is recommended that the fluid loss from a water-based drilling mud should not exceed 15 mL/30 min under the standard API test conditions.[80] Accordingly, despite that the addition of copper nitrate and KMnO4 increases the fluid loss of the base drilling mud, the 30 min fluid loss extents are still below the capped limit.

Figure 9

API fluid loss of base and the scavenger-containing drilling muds.

Conclusions

KMnO4-containing drilling mud possesses an excellent H2S scavenging performance. This drilling mud has also more favorable rheological properties compared to copper nitrate-containing and ZnO-containing drilling muds. The latter two drilling muds did not significantly alter the rheology of the base drilling mud. In terms of fluid loss characteristics, despite the increase in the fluid loss upon the addition of copper nitrate and KMnO4, the extents of the encountered fluid loss are still within the recommended range for all the tested scavengers. The H2S scavenging performance of copper nitrate-containing drilling mud, despite being more than twofold lower than that of the KMnO4-containing drilling mud, is still satisfactory (it is more than four times higher than the H2S scavenging capacity of ZnO-containing drilling mud). The superior H2S scavenging capacity of the KMnO4-containing drilling mud and its favorable rheological properties suggest that KMnO4 addition to water-based drilling mud formulations would be beneficial. However, further studies are required to elucidate the compatibility of KMnO4 with other drilling mud additives, particularly under high-temperature and high-pressure conditions.