Developed along Zagros Fold-Thrust Belt, Gachsaran Formation hosts important petroleum traps and its depositional environment has been researched by many researchers [1C.A.E. O’Brien, "Salt diapirism in South Persia", Geol. Mijnb., vol. 19, pp. 337-376.-5A. Bahroudi, and H.A. Koyi, "Tectonic-sedimentary framework of Gachsaran Formation in the Zagros foreland basin", Mar. Pet. Geol., vol. 21, pp. 1295-1310.
[http://dx.doi.org/10.1016/j.marpetgeo.2004.09.001] ]. The formation has been divided into seven members, namely Member 1 through 7. Member 7 consists of limestone, gray marl, and anhydrite. Member 6 involves a succession of anhydrite, limestone, red marl and rock salt. Member 5 is composed of an alternation between gray and red marls, anhydrite and thinly limestone layers. Member 4 contains thick salt layers alternating with anhydrite, gray marl, and seldom limestone. Member 3 includes anhydrite and thick marl layers. Member 2 comprises rock salt, gray marl, anhydrite, and laminated limestone. Hence, Member 1 is made up of five different evaporitic cycles of marl, anhydrite, bituminous shale, and limestone, with a typical width of 50 m. The latter member covers the Asmari reservoir in the southwest of Iran [6A.S. Alsharhan, and A.E.M. Nairn, Sedimentary basins and petroleum geology of the Middle East., Elsevier: Amsterdam, .]. The width of the sedimentary formation varies across the area, as shown in Fig. (1), but total width of the complete series has been approximated to be 1600m [7H. Motiei, Geology of Iran, Stratigraphy of Zagros., Geology Organization of Iran: Tehran, .]. Several scenarios have been developed to relieve wellbore closure while drilling salt layers. For example, the traditional strategy has been to drill quickly through the salt and to carefully keep the drilling fluid cool to postpone creep [8M.B. Dusseault, V. Maury, F. Sanfilippo, and F.J. Santarelli, Drilling through salt: Constitutive behavior and drilling strategies, American Association of Rock Mechanics, North American Rock Mechanics Association, 04–608, pp. 1–13, 2004.]. Reaming and jarring may also be needed in such situations [9C. Chatar, S. Mohan, and M. Imler, "Overcoming a difficult salt drilling environment in the Gulf of Mexico: A case study", Paper IADC/SPE 128192 presented at the IADC/SPE Drilling Conference and Exhibition, New Orleans, LA, USA, pp. 1-12.]. Oil-based Mud (OBM) inhibits wellbore dissolution but cannot decelerate the wellbore closure unless it is kept cool enough during the circulation [10M.B. Dusseault, M.N. Gray, and P.A. Nawrocki, "Why Oil wells Leak: Cement behavior and long-term consequences, SPE 64733 presented at the SPE international oil and gas conference and exhibition", Beijing, China, .]. Under-saturated Water-based Mud (WBM) presents a great alternative for penetrating into thin and thick salt layers to mitigate the creep and wellbore closure. Most frequently experienced salts along Gachsaran Formation include halite (NaCl), tachyhydrite (CaCl₂.2MgCl₂.12H₂O) and anhydrite (CaSO₄). Normally, deposition accumulation follows an opposite trend to that of salt solubility in water. For the salts comprising the Gachsaran Formation, the order of solubility is as follows: tachyhydrite > halite > anhydrite. The formation contains different types of evaporitic salts, including sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl₂), and magnesium chloride (MgCl₂). Accordingly, one may develop a formulation for the drilling fluid by which the dissolution can be avoided. Such a synthetic fluid is preferable over WBM as it is free of water and interferes interfering the salts solubility. With such a synthetic mud, the well caliper is commonly within gauge when compared to the case with saturated water-based fluid. Alternatively, drilling highly flexible evaporites with a synthetic mud may cause wellbore collapse, reaming problems, high torque requirements, well deviation, stuck pipe, and casing collapse. In some instances, the stress developed upon drilling a salty formation caused a stuck bit, mainly through the connections, necessitating the injection of fresh water to have the bit released [11R.F.T. Lomba, G.T. Teixeira, R.R. Pessanha, B.S. Lomba, and M.G. Folsta, "Cardoso Jr, and Gonçalves J. T. lessons learned in drilling pre-salt Wells with water based muds, petrobras", Offshore Technology Conference, .].

Fig. (1) Stratigraphic correlation of Cenozoic formations within the Zagros Fold-Thrust Belt [12G.A. James, and J.G. Wynd, "Stratigraphic nomenclature of Iranian oil consortium agreement area", AAPG Bull., vol. 49, pp. 2182-2245.].

In cases where a stream of saltwater flows beneath the salt, there are chances that the mud density decreases upon reaching the base. Thus, it should be kept in mind that, when saltwater is used to drill a salt formation, the salt is not seen as a contaminant, while it is recognized as a contaminant with freshwater muds. Chaney P.E [13P.E. Chaney, "A Review of recent advances in drilling mud control, Drilling and Production Practice", American Petroleum Institute, pp. 31-46.] explained that, besides solid salt particles and salt water, seawater has been recognized as a significant source of contaminants as it contains magnesium (Mg²⁺) and calcium (Ca²⁺) ion along with clay minerals. Ca²⁺ and Mg²⁺ ions in seawater are harmful to WBMs. Indeed, at higher pH values, magnesium and calcium hydroxide are relatively insoluble in water, and application of caustic soda to eliminate magnesium further inhibits calcium dissolution and lime sedimentation. Kumapayi [14Y. Kumapayi, K. Bello, O. Adekomaya, A. Akintola, J. Dala, I. Mohammed, and O. Olafuyi, "Investigating the Effects of Contaminants on the performance of Oil-Based Invert Emulsion Drilling Fluid", Pet. Technol. Dev. J., vol. 2, pp. 60-74.] explained that “the Gulf of Mexico seawater requires 4.3 to 5.7 kg/m³ of caustic soda to precipitate all magnesium. In seawater, the preferred treatment for magnesium removal is caustic soda, while the preferred treatment for calcium removal is soda ash”. Ali et al. [15K. Ali, C. Vipulanandan, and D. Richardson, "Salt contamination on the resistivity and plastic viscosity of a bentonite drilling mud", Proceedings of CIGMAT Conference & Exhibition, .] looked into the result of NaCl contamination on rheological properties of bentonite-containing mud, concluding that both Plastic Viscosity (PV) and Electrical Resistivity (ES) decrease with a rise in salt content. Basirat et al. [16B. Basirat, C. Vipulanandan, and D. Richardson, "Equivalent resistance of a 3-D conductive medium", Proceedings of THC-IT Conference & Exhibition, .] carried out similar research and explained that once contaminated, drilling muds tend to exhibit approximately 30% higher fluid loss along with 86% lower electrical resistivity. Furthermore, Hassiba and Amani [17K J. Hassiba, and M. Amani, The effect of salinity on the rheological properties of water based mud under high pressures and high temperatures for drilling offshore and deep wells earth science research; Vol. 2, 1 2013.] demonstrated that salt contaminants tend to escalate shear stress/shear rate while KCl contaminants tend to reduce the shear stress/shear rate in WBM. According to Neff [18J.M. Neff, Composition., Environmental fates and biological effect of water based drilling mud and cuttings discharged to the marine environment, p. 17.], the amount or quantity of solid particles, such as drill cuttings and formation solids, in the mud vary with a grain size of the rock which is being drilled. Amani et al. [19M. Amani, and J.H. Khaled, "Salinity Effect on the Rheological Properties of Water Based Mud under High Pressures and High Temperatures of Deep Wells, SPE-163315-MS presented at SPE Kuwait International Petroleum Conference and Exhibition, 10-12 December", Kuwait City, Kuwait, .] investigated the effect of a flow of magnesium-containing saltwater on the performance of drilling mud at elevated temperature in an attempt to evaluate the mud response in down-hole conditions. They showed that, such a flow tends to lower the performance of hematite and barite-containing muds; the finding suggested that saltwater influences the dispersion, hydration, and flocculation behaviors of the weighting and viscosifier agents, creating particles dispersion and increasing the number of specific platelets in the suspension system, thereby making the mud inadequate for carrying the cuttings. However, ferrobar mud could retain the cutting in the suspensions with a flow of magnesium saltwater at enhanced temperature. Adekomaya O.A [20O. Adekomaya, "Olufemi. A., Olalekan O. an experimental study of the effect of contaminants on the flow properties of oil-based drilling mud", J. Pet. Coal, vol. 53, no. 4, pp. 315-319.]. and Sami N.A [21N. A. Sami, "Effect of magnesium salt contamination on the behavior of drilling fluids", Egypt. J. Pet., .]. reviewed previous studies and discovered that rheological and filtration properties (and hence efficiency) of drilling fluid are damaged by magnesium ion contamination, as indicated by reduced Plastic Viscosity (PV), Yield Point (YP), and gel strength. Also, magnesium ion was found to increase filtration loss volume of the drilling fluid. Indeed, with increasing the concentration of magnesium chloride salt, the fluid loss into the formation raises. Most of the works reviewed above-examined clay-containing or low-density WBMs, while Gachsaran Formation is commonly drilled with heavy-weight salt-saturated mud with high sensitivity to treatment and maintenance operations. The present study begins with identifying symptoms leakage of Gachsaran formation fluid into drilling mud. Secondly, the effects of the contamination resulted from such leakage on the drilling fluid are examined in laboratory based on the amounts of different contaminants. Finally, a discussion is presented on the exposure, treatment, and maintenance methods required to continue the drilling operation at minimum damage/cost.

Soda ash is normally used to precipitate calcium. Accordingly, 2.85 kg of soda ash per m³ of fluid will precipitate 1078 mg/l of calcium. To maintain a constant pH while precipitating CO₃ and HCO₃, when pH of the mud exceeds 10.5, the entire deal of magnesium has been essentially precipitated as magnesium hydroxide. Small daily additions of lime will be required for this purpose. pH of the mud should be maintained between 11.0 and 11.5 to keep most of the existing carbonates in the mud in the form of CO₃ rather than the more adverse HCO₃. By maintaining the pH value using lime and caustic soda, the total content of carbonates can be minimized. Table 6 showed that pH adjustment with lime and caustic soda may be helpful for continuing the drilling, although the drilling fluid properties were not recovered satisfactorily.

Table 6
Contaminated mud treated with caustic soda and lime.

When formation fluid (water) leaked in well and the drilling fluid contaminated, mud properties such as Plastic viscosity, Appearance viscosity, Yield point, Gel strength, Cake thickness, total hardness and fluid loss increased, additionally the pH is decreased.

Prevention has been proved to be superior to cure. When the formation fluid leakage symptoms are observed in a drilling fluid, mud weight must be increased to stop the leakage. Both pre-treatment and regular checking of the mud in the mud tanks help identifies mud contamination promptly. However, the following can be used to treat and maintain contaminated drilling fluids:

• Adding caustic soda to raise pH beyond 10.5 and have Mg²⁺ precipitated.
• Adding soda ash to eliminate Ca²⁺ according to the proposed formula.
• Adding lime to prevent contamination resulted from HCO₃- and CO₃-.

A number of methods have been proposed to treat the contaminations, as mentioned in sections of drilling handbooks on drilling fluid maintenance, including the use of gypsum- or carbonate-based mud. However, such methods work light-weight muds for clay rather than heavy-weight salt-saturated muds [27M.R. Annis, and V.S Martin, Drilling fluids technology., Exxon Company: U.S.A, ., 28Drilling Fluids Reference Manual, Baker Hughes., Revision, .].

• Lignosulfonate can be added to improve mud rheology. Increased contents of calcium, magnesium, carbonate and bicarbonate ions and decreased value of pH were identified as symptoms of formation fluid leakage into drilling fluid in Gachsaran Formation. The best treatment was found to be prevention measures.
• Adding low amounts of lime to the mud pit to control its pH value under 11.
• Moreover, caustic soda and lime can be added to retain pH value between 10.5 and 11.5; over-treating with soda ash for removing Ca²⁺⁺ may increase the contents of carbonate and bicarbonate ions in drilling fluid.