Mechanisms Controlling Filtration at the Core Bit for Oil-Based Muds
- H.R. Warner Jr. (Arco E&P Technology) | J.J. Rathmell (Arco E&P Technology)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 1997
- Document Type
- Journal Paper
- 111 - 118
- 1997. Society of Petroleum Engineers
- 1.11 Drilling Fluids and Materials, 1.6.9 Coring, Fishing, 1.10 Drilling Equipment, 5.2 Reservoir Fluid Dynamics, 5.3.4 Reduction of Residual Oil Saturation, 5.2.1 Phase Behavior and PVT Measurements, 1.2.3 Rock properties, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.6 Drilling Operations, 4.3.4 Scale
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Laboratory half-scale coring experiments using 4-in.-diameter reservoir rock samples were conducted with field oil-based muds (OBM). The OBM were formulated to have low water content and low fluid loss to minimize the potential impact of the invading OBM fIltrate on connate water saturations (Sw). The OBM had been used previously during Prudhoe Bay (Alaska) field coring operations conducted to obtain connate Sw measurements. Laboratory overbalance pressure and temperature were similar to the reservoir downhole environment. The coring test data were OBM filtrate volume generated during coring, pressure in the rock beneath the bit as the bit approached, and pressure in the half-scale 2-in. core during and after the core bit passed by. Also, at the end of each test, dynamic and static OBM filtrate volumes were measured while only circulating the mud and with no mud flowing. OBM filtrate properties, viscosity, emulsifier content, water content, and interfacial tension (IFT) were measured after coring was concluded. Sw was measured before and after coring.
Conclusions from these tests are as follows.
1. The pressure measurements show that the OBM filter cake controls the pressure gradient both in the core and in the rock directly beneath the bit.
2. The pressure gradient across the OBM filter cake took up almost all the pressure difference between the mud and formation pressures, even at the face of the core bit.
3. The OBM filtrate contains approximately 10% of the carboxylate soap and water found in the whole OBM. OBM filtrate IFT is about half the reservoir oil/brine IFT. The invading OBM filtrate properties are similar to reservoir crude.
4. The water-concentration increase in the OBM was approximately the amount expected as a result of freeing water from the rock drilled up during coring.
5. Because of Sw variations within the rock samples, core sample Sw measurements could not be used to quantify any potential water movement in the core caused by OBM filtrate invasion.
These results mean that the pressure gradients in the core during coring are very low, so displacement of water by capillary number effects will not occur, and above the oil/water transition zone, cores taken with OBM similar to the ones used here should yield reliable in-situ Sw values.
In oil and gas reservoirs, Sw measurements on rock samples taken from cores cut with OBM are typically considered to be reliable and quantitative measures of the in-situ Sw values. They are also indicative of the vertical Sw distribution throughout the hydrocarbon column, except in the oil/water transition zone.1-3 When using OBM core Sw values quantitatively, one assumes that neither the OBM filtrate invasion during coring nor the depressuring during core surfacing has altered the in-situ Sw through loss of water from the core. A change in Sw may occur because of loss of gas and decreased temperature, but the surface value of Sw is quantitatively adjusted through formation volume factor to the in-situ value. This paper describes an extensive laboratory study undertaken to understand the effect of OBM filtrate invasion on core Sw . A companion paper discusses the surfacing aspects of OBM coring operations on in-situ Sw. General Prudhoe Bay field conditions are not discussed here because they are described in the companion paper.4 Several other recently published papers describe Sw variations throughout the Prudhoe Bay field.5-7
There are a few previous studies of this issue in the technical literature but none have been conducted as close to downhole coring conditions as presented here. These include the study by Jenks, et al. where diamond microbits and water-based muds were used to core sandstone samples to determine the feasibility of coring for residual oil saturations,8 and a small follow-up experimental study by the same authors concerning OBM coring to determine in-situ Sw.9 These authors concluded that residual oil can be stripped from the rock close to a working bit under conditions frequently encountered in coring, and that coring for immobile water saturation is more reliable than coring for residual-oil saturation, because connate water is held as hydraulically isolated volumes retained in finer pores by capillarity. Pressure gradients in the rock, determined from three pressure taps placed perpendicular to the drilling direction, were 900 to 1,500 psi/ft. By comparison, in our study all pressure gradients were about one-hundredth of these values because the OBM filter cake was found to absorb effectively almost all (98%) of the pressure differential between the mud system and the rock pore system.
A different approach for studying this issue was pursued by other researchers.10 Assumptions were made about the composition of the mud filtrate, the pressure gradient in the rock ahead of the bit, and the volume of OBM filtrate invading the rock. These assumed values were then used as input conditions for a series of linear corefloods to determine whether any connate water would be mobilized by flooding at these conditions. As is shown in our study, all of these assumptions were far more severe than would occur during field coring operations using OBM, and show the problem with presuming values for any or all of these parameters without proper preliminary testing.
In this paper, which presents the key aspects of four sets of experiments run over a period of 3 years, the experimental procedures and materials are discussed, the results of several of these coring tests are shown, the various complementary technical analyses and laboratory studies are reviewed, and the summary findings are presented. The authors had the opportunity to analyze one set of experiments, recognize their shortcomings, and then design the next set. The final set, therefore, proved to be the most meaningful for addressing the key technical issues concerning potential OBM filtrate invasion effects on connate Sw. A total of about 20 coring tests were conducted.
Rock and Fluid Properties
Preparation and Handling of Rock Samples.
Two types of core material were used in these experiments. First, Prudhoe Bay Ivishak sandstone 4-in. whole core samples were prepared for most of the coring tests and other experiments. These included as-received OBM preserved whole core pieces (four tests) and, in many tests, extracted whole core samples. Second, in two tests, Boise or Massillon outcrop sandstone slabs were cut into 4-in. cylindrical samples for testing.
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