Estimating Mechanical Properties of Shale From Empirical Correlations
- Per Horsrud (SINTEF Petroleum Research)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2001
- Document Type
- Journal Paper
- 68 - 73
- 2001. Society of Petroleum Engineers
- 6.1.5 Human Resources, Competence and Training, 5.1.5 Geologic Modeling, 1.5 Drill Bits, 1.6.3 Drilling Optimisation, 1.12.1 Measurement While Drilling, 5.1 Reservoir Characterisation, 5.6.1 Open hole/cased hole log analysis, 5.3.4 Integration of geomechanics in models, 1.6.9 Coring, Fishing, 1.6 Drilling Operations, 4.3.4 Scale
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Borehole-stability prediction requires knowledge of the mechanical properties of the formations which is rarely available in shale sections. This paper presents empirical correlations to assist in predicting shale mechanical properties. The correlations are based on extensive laboratory testing of shale cores primarily from the North Sea. The acoustic P-wave velocity is a primary input parameter in several of the correlations; thus, various sources of the P-wave velocity such as sonic wireline, sonic measurement while drilling (MWD), and acoustic measurements on cuttings may be used to obtain somewhat continuous estimates of shale mechanical properties. Borehole-stability evaluations can be made at different stages in the drilling process (planning, while drilling, and post-analysis). Other applications of the correlations where shale mechanical properties are required are evaluation of overburden compaction during depletion and optimization of the drilling process (selection of bit type, bit parameters, etc.).
The properties and response of shale are important for the petroleum industry in basin modeling, interpretation of seismic response, and drilling with respect both to potential borehole-stability problems and drillability. However, the shales are not the primary target; therefore, shale samples (cores) from deep boreholes are scarce because of the additional cost related to coring operations in deep boreholes.
Stability problems during drilling may be very costly. Drilling of long sections at high angles in shales may represent a considerable challenge. This has motivated operators to core and test potentially troublesome shales in the overburden.1 Coring, however, can provide only discrete data points for use in a stability evaluation and can cover only a very limited depth range. There is an obvious need for methods that can provide shale properties both on a more continuous basis and at less cost. Such methods can be based on different information sources (e.g., wireline logs, MWD, and drill cuttings). Static mechanical properties are not measured directly by any of these tools. More or less empirical correlations have been used extensively in sandstones and to some extent in shales and mixed lithologies.2-5 Publicly available correlations often suffer from an overrepresentation of strong rock samples, which is not ideal for a stability evaluation. Further, they are based frequently on published data from different sources, where it is difficult to control the consistency of test material, test procedures, and data interpretation.
This paper presents correlations which have been developed from a dedicated testing program on a number of shale cores, mainly from the North Sea. The testing program included measurements of a wide range of petrophysical and mechanical properties. Several of the shales were potential candidates for instability problems; thus, weak shales also are represented in this data set.
Procedures and Apparatus for Triaxial Testing of Shales
Shales have certain characteristic features that make them difficult to handle correctly. The two most important characteristics are low permeability6-9 and sensitivity to contacting fluids. Special precautions must be taken to preserve the core,1 and special conditions must be applied during laboratory handling and testing.2,6,10 The shale is unloaded from pressure and temperature, and this may cause damage and alterations in several ways (e.g., creation of microcracks, disking, and reduced saturation caused by expansion). A shale taken from a deep borehole may not be 100% saturated under atmospheric conditions. This makes the laboratory testing susceptible to artifacts10,11 unless special procedures are applied. Because of the low permeability of these shales, testing of mechanical response can be very time-consuming and thus relatively expensive.
The field cores that were tested were all well preserved so that loss of pore water after coring was prevented. Test samples were drilled from the core with the sample axis normal to the apparent bedding plane. The samples used for triaxial testing were 11/2 in. in diameter and approximately two times longer than the diameter. The triaxial tests were run as consolidated-undrained (CU) tests, a commonly used type of test for low-permeability shales. The tests consisted of the three segments illustrated in Fig. 1: loading to a predetermined level of confining pressure and pore pressure; consolidation (i.e., a period of constant confining pressure and drainage of the pore fluid against a constant pore pressure); and undrained axial loading under a constant axial displacement rate until failure of the sample. In this last phase, the pore pressure will increase because of the undrained boundary condition.
Internal instrumentation of the test sample is shown in Fig. 2. In addition to measurement of external load, pressure, and deformations, the pore pressure at both ends of the sample and acoustic-wave trains in both the axial and radial directions are recorded.
The time required to run a test depends on the permeability of the shale. Concepts from soil-mechanics testing can be applied to determine when consolidation is completed, and also to determine the appropriate displacement rate in the undrained part of the test,12 to make sure that pore-pressure equilibration is ensured throughout the sample.
Shale Testing and Database Establishment
Table 1 gives an overview of the field cores that have been tested and some key parameters for these shales. Some outcrop clays/mudstones included in the database13 and in the basis for the correlations are included. Table 1 shows a considerable spread in the properties of the shales. For Tertiary shales, porosity is generally high, with 55% porosity for the shallowest shale. As expected, porosity decreases with increasing depth and age, down to 3% porosity for the deepest shale.
Triaxial testing is the primary basis for the empirical correlations developed. This paper will present how the shale properties of the correlations have been determined. At the initiation of the project it was difficult to anticipate which parameters could be correlated. An extensive test and characterization program was carried out for each core. This included additional testing and characterization such as mineralogy (X-ray diffraction), petrographic description using scanning electron microscopy (SEM), water content and bulk density, specific surface area, cation exchange capacity, pore size distribution, and permeability. For details about this and a more in-depth discussion of the shales, the reader is referred to another publication.6
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