New Laboratory Tests Evaluate the Effectiveness of Gilsonite Resin as a Borehole Stabilizer

Abstract

Summary. Gilsonite resin is a naturally occurring, mined carbonaceous material classified as an asphaltite. For many years, Gilsonite has been used in drilling fluids as an additive to assist in borehole stabilization. It has been well documented that this material works efficiently to minimize hole collapse in unstable shale sections. However, because Gilsonite is an asphaltite with a high-temperature softening Point, duplication of its mending action in laboratory tests conducted at ambient temperatures and low pressures has been difficult. New laboratory techniques have been developed only recently to evaluate Gilsonite under simulated downhole conditions. With innovative procedures and a newly built downhole simulation cell (DSC), we tested Gilsonite under temperatures and pressures similar to drilling conditions in which the product would normally be used. These tests indicate that borehole enlargement was minimized by use of Gilsonite while substantial enlargement was measured when the same drilling-fluid system was used without Gilsonite. Another new test procedure was developed to discriminate between various types of Gilsonite products and to determine the most effective product under different temperature and pressure environments. This procedure made use of a high-temperature, high-pressure (HTHP) fluid-loss cell with Berea cores as a filtering medium. Scanning-electron-microscope (SEM) examination of the test cores from both testing procedures provided insight into the mechanisms of how Gilsonite provides stability under downhole conditions. Field data from wells drilled in widely different geological environments support the conclusions reached from laboratory tests. Introduction For many years. Gilsonite and other asphaltic-type products have been used in water-based drilling fluids as additives to assist in borehole stabilization. It has been well documented that these additives can minimize hole collapse in formations that contain water-sensitive, sloughing shales. The causes of borehole instability are numerous. The reasons for tile instability can be mechanical. chemical, or physical in nature. The mechanical problems include borehole erosion by high annular velocities, adverse hydraulic stresses caused by high annular pressures, hole collapse from high swab and surge pressures because of excessive wall cake, and stressed erosion resulting from drillstring movement. Chemical alteration problems include hydration, dispersion, and disintegration of shales because of the interaction of clays with mud filtrate. Physical instability problems include the spalling and rock bursts of shales caused by subnormal pressure or overpressure relationships of hydrostatic and formation pressures. Fracture and slippage along bedding planes of hard, brittle shales and the collapse of fractured shales above deviated holes are also physical problems encountered during drilling of troublesome shales. This problem also occurs in non-deviated holes during drilling of overpressured shales. Borehole instability problems are often referred to as sloughing, heaving, spalling, or overpressured shales, mud balls, mud rings, and many other descriptive names. This problem has many solutions. The use of additives to inhibit entirely or partially the swelling of clay has been well documented. The adjustment of hydraulic conditions is another solution to reduce mechanical alteration. Knowing and controlling the pore pressure of the problem formations is used often. In this paper, the use of Gilsonite and asphaltic-type additives to minimize physical and, to some extent, chemical alterations. will be discussed. The effectiveness of Gilsonite and other asphaltic-type products in the laboratory has been difficult to evaluate because most test procedures are performed at ambient temperatures and low pressures. Because Gilsonite and some asphaltic-type products require temperature and pressure to be effective, the results of these tests are skewed toward those additives that control shale problems by chemical reaction. These tests do not compare "apples with apples," but "apples with oranges." Equipment has been designed recently to study drilling-fluid interaction with formation rock in the laboratory under simulated downhole conditions. One procedure has been used to evaluate the effects of various water- and oil-based mud systems on shale stability under downhole conditions by use of the DSC. This joint industry project, DEA Project 22, was sponsored by the Drilling Engineering Assn. A similar project now being conducted, DEA Project 38, studies performances of asphaltic-type products and Gilsonite in various muds and on different types of shale. Both projects contain proprietary information and will not be discussed in this paper. This paper discusses the results of our independent series of tests on Gilsonite. Gilsonite is a naturally occurring, solid carbonaceous material that is classified as an asphaltite. It is a relatively pure hydrocarbon without significant amounts of mineral impurities. Gilsonite has a softening point of approximately 370F [188C], although a lower softening point, 330F [166C], is available. Other asphaltic-type additives, including air-blown asphalts and sulfonated asphalts, have softening points higher than 240F [116C]. Some of these additives have been treated with a surfactant to provide better water dispersibility or sulfonated to provide various degrees of solubility. According to the suppliers of Gilsonite and other asphaltic-type products, these additives are used to help control sloughing shale problems by minimizing shale slippage along microfractures or bedding planes by physically sealing and plugging. We initiated an apples-with-apples study of Gilsonite and asphaltic-type products. Products were evaluated by use of existing testing procedures under ambient temperatures-e.g., triaxial testing for shale stability, lubricity evaluation, and effects on filtration control. The results of the initial test series indicated that a more in-depth evaluation was needed under conditions in which the additives begin to function properly. Several new test procedures that included the use of an altered HTHP fluid-loss cell and a DSC were designed. The results of these tests indicate that these procedures can be used effectively to evaluate and to discriminate between Gilsonite and asphaltic-type products. In addition, the use of the DSC allows the user to discriminate or to rank the effectiveness of Gilsonite and asphaltic-type additives with additives that attempt to minimize borehole instability through chemical inhibition. These procedures allow the user to compare not only apples with apples, but also apples with oranges. Theory Gilsonite and asphaltic-type materials have been used for many years to stabilize sloughing shales and to reduce borehole erosion. It is proposed that the material, added to a mud system before a problem shale is encountered, would penetrate the shale pore spaces, microfractures, and bedding planes as the bit penetrates the formation. By a plastic-flow mechanism, Gilsonite would extrude into the pores, fractures, and bedding planes to reduce or to minimize filtrate and whole mud invasion, and thus bond the matrix to prevent sloughing.