Recent developments in energy exploration at depths of 5000 to 25,000 feet have made it necessary to quickly and reliably determine the in-situ stresses acting on the wellbore.
Differential Strain Analysis (DSA) is being investigated as a technique applied to core samples indirectly determine the in-situ stress state.
Testing is being pursued in three steps. First, field measurements of strain are made in-situ as the core is pulled out of the well. Second, the cores are brought to the lab and DSA is performed under in-situ hydrostatic conditions. Third, the rock is examined microscopically.
These tests have been performed on both oriented and non-oriented cores from Texas, Louisiana and Pennsylvania.
At this point in the investigation, it appears very favorable that a reasonably accurate estimate of the 3-dimensional stress state can be obtained usingthe strain curve analysis method. It has been demonstrated mathematically chat not only the ratio of the stresses can be derived but also the orientation of the stresses in free space. The application of these equations to the data from the latest high quality runs yields results well within the reproducible tolerance of other methods.
Recent trends in the oil and gas industry have to been toward developing reservoirs that were previously passed over due to one reason or another. One type ofthese reservoirs is the "tight" gas or oil formations. The development of advanced fracturing techniques has allowed commercially viable production rates from these formerly low producers. However, even with these fracturing techniques, much of the reservoir may be leftunrecovered because of the unique drainage pattern in low permeability reservoirs. Smith 1(1979) has demonstratedthat the drainage around a fracture-stimulated well is elliptical rather than radial. This is even more pronounced as the fractures increase in length, as in massive hydraulic fracturing. Consequently, well spacing based on radial drainage may provide inefficient recovery due to overlapping drainageat fracture tips and "dead" areas between fracture flanks. If there was a way of predicting the fracture orientation when planning the spacing of wells in a developing field, recoveries could be increased to recover an additional 30% of the total reserve (Smith 1, 79). That would greatly increase the realized potential from many of the giant gas fields.
Considerable research has been devoted to measuring fracture propagation during the fracturing process. The various methods used include: tiltmeter surveys, acoustic emission surveys (both surface and well-bore), resistivity surveys, etc. The major restriction on the fracture propagation surveys is depth. Resolution drops off rapidly as depth (and therefore, distance from sensors) increases. Another attempt at predicting fracture orientation has been to measure directly the in-situ stress field, since this is the primary factor influencing the c09trol of fracture orientation (Warpinski, et al.2 1980; Hubbert and Willis3, 1956). The traditional method of determining stress in rockmasses has been by the over coring process (Obert and Duval4, 1967).