Summery
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 stabilizer. 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 stabilizer. 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 drill string 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 the drilling of troublesome shales. This problem also occurs in non-deviated holes during drilling of over pressured 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. Evaluating 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 micro-fractures 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.
Testing Procedures
It has been difficult to duplicate the effectiveness of Gilsonite as a borehole stabilizer in the laboratory, as seen in the field. Several companies have published data conducted at ambient temperatures and pressures. Many of the tests referenced were or are designed for additives which stabilize shale or clay by partial inhibition or encapsulation. Since Gilsonite and the insoluble blown asphalts require both temperature and pressure for extrusion to occur, these additives do not compare favorably in ambient, low-pressure tests. Therefore, to effectively evaluate Gilsonite and asphaltic materials, tests must be designed using higher temperatures and pressures.
The first test, designed and run by Chevron, used existing laboratory equipment. A NL Baroid High-Temperature/High-Pressure fluid loss cell was modified to use a thin 0.5 inch Berea sandstone core as the filter media. (appendix 1) Gilsonite or asphaltic type materials to be tested were dispersed into the base mud slurry. The mud was then placed into the cell, the temperature, and pressure adjusted to specific parameters, the test run, and the filtrate collected and measured. Upon completion of the test, the core was removed. After cooling, the core was sliced and examined under a high-powered microscope. The depth of invasion was measured. Using the same slurry composition, additional tests were performed in which the temperature and pressure were increased to determine effective temperatures at which Gilsonite or the asphaltic materials would extrude into the void spaces of the core and whether a thin, inter-matrix filter cake would develop, or continue to further invade the core. This test is useful to discriminate between insoluble manufactured products such as Gilsonite with different softening points, blown asphalts, and soluble sulfonated blown asphalts. The test can be used to select additives that seal off shale micro-fractures and form a plating film on the face of the borehole.
Following the series of tests which used the HTHP filter press/Berea core, Chevron initiated a study of Gilsonite using the Downhole Simulation Cell (DSC). The apparatus was designed and built by TerraTek Inc., Salt Lake City, under the direction of the O’BrienGoins-Simpson and Associates, Houston. The DSC is a triaxial testing apparatus which can drill through a shale core as large as 6.5 inches in diameter while circulating fluids at temperatures up to 350°F. A schematic of the cell is shown in Figure 1. The complete description of the apparatus can be found in the Simpson, Dearing, Salisbury paper.
For our purposes, the Pierre shale was used. Table 1 shows a complete mineral analysis of the Pierre Shale core. Table 2 shows the composition of the circulating fluid and the Gilsonite additive, and Table 3, the testing parameters. The first test run confirmed that the DSC could evaluate the effectiveness of Gilsonite under simulated drilling conditions. After this initial success, a series of tests were designed to focus on Gilsonite’s effectiveness under varying temperature and pressure conditions. The investigation also included an evaluation of pretreatment versus treatment after instability occurred and an evaluation of low, normal, and high softening point Gilsonites. The results of this testing program, which will be discussed later in the paper, showed the DCS to be a very effective apparatus for evaluating borehole.
Conclusions
Based on the results of the testing program our company has conducted and comparing these results with some well histories, the following conclusions are offered.
1. To monitor the effectiveness of a Gilsonite or asphaltic-type product, laboratory tests should be conducted under simulated downhole conditions. One of the most effective laboratory apparatus is the Downhole Simulation Cell.
2. There are differences in Gilsonite additives as to their effectiveness at varying temperatures and to some degree pressure. A simplified high-temperature high press filtration cell using a Berea core as a filtering medium can be used to discriminate Gilsonite products and asphaltic type products concerning plugging ability.
3. Gilsonite is an effective additive as a Borehole Stabilizer. The additive plugs off microfractures, bedding planes, and pore spaces, and deposits a thin film on the borehole wall which mitigates hole erosion. It is recommended that Gilsonite be added to a system prior to drilling shale sections where borehole instability is expected. However, laboratory tests do indicate that treatment after destabilization has been initiated to be effective with minimum concentration values of 3-4 pounds per barrel.
Our company concludes that the downhole simulation cell can be used to evaluate additives which stabilize troublesome shales either through a mechanical action such as Gilsonite or a chemical action such as inhibition. Under these test conditions “apples can be compared with oranges”.
