Abstract

Thermal and mechanical rock properties were evaluated with special laboratory tests and logs to analyze a Steam Assisted Gravity Drainage (SAGD) project by MARAVEN S.A. (now part of PDVSA Exploración y Producción) in the heavy oil TÍa Juana field on the eastern side of Lake Maracaibo, Venezuela. This SAGD project is the first application of this technology in Venezuela and consisted of two parallel horizontal wells on top of each other where steam is injected in the upper well and oil is produced in the lower well. Special coring equipment and procedures were utilized in a specific well to obtain a low perturbation core. Dipole sonic logging was successfully used to obtain a continuos profile of P and S wave velocities in these extremely loose unconsolidated sands to serve as calibration with the laboratory tests. A special series of laboratory tests were carried out in this core to determine thermal and mechanical properties which will be useful to characterize these extremely loose unconsolidated sand reservoirs under the varied conditions of temperature and pressure encountered in this project. Basic thermal properties such as thermal conductivity, specific heat and thermal expansion were specifically determined with modern equipment and techniques for the first time in Venezuela. Triaxial strength tests were carried at 72°F and 330°F to see the effect of high temperatures due to steam injection on stress-strain behavior, modulus and friction angle. This reservoir characterization at high temperatures was fundamental in analyzing problems of borehole stability during drilling as well as production in the injector and producer wells because of the mutual interaction due to the proximity of both wells. Of specific importance was the limiting flow rate of the steam injection well to eliminate hydraulic fracturing and the limiting production rate of the producing well to eliminate sand production and stability problems. The interaction and stability of both holes was evaluated with a fully coupled finite element program using Biot's poroelastic theory. This program was used to determine stability, stress and pore pressure values around the injector and producer wells under steady state conditions. The geomechanical analysis helped in defining the safe operating limits for the dual horizontal wells was useful to explain the behavior of some SAGD wells already in production in Tia Juana field and to plan for future ones.

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