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A geomechanics based methodology was utilized to analyze sand production problems as well as the amount of damage caused to the wells of the Southern Area 2 sector of the Ceuta field in Lake Maracaibo, Venezuela. Sand production in this field was not originally expected since the reservoir is composed of a well consolidated sand at depths larger than 16,000 feet and with rocks having an unconfined compressive strength larger than 6000 psi. Geomechanically speaking, sand production is caused when the destabilizing stresses on the formation are larger than the resistance due to formation strength. However, certain well operations can affect the destabilizing stresses as well as the formation strength. Therefore geomechanics must be used together with proper operational procedures because many operational details can also have a profound effect on the occurrence of sand production. One of the most important factors has to do with the production practices involving proper choke sizes and excessive choke changes. Sand production data gathered in the Ceuta field was analyzed using a geomechanical framework of understanding to predict well damage and problems during choke changes. The data defines 3 different stages of sand production for wells in this field. Each stage has a particular behavior and different problems associated with them. The sand production data was integrated together with the fluid production data to calculate the amount of accumulated sand production. A simple spherical model was developed to calculate cavity size around the casing utilizing the accumulated sand production. It was found that a well could produce large quantities of sand before total well failure. Total well failure appears to be related to the size of the cavities around the casing which leave it unsupported and also to erosive damage to the casing done by the sand laden fluids. The sand producing data was then used to diagnose the stage at which the sand production problem occurs and to change choke utilization practices in order to minimize sand production. Special laboratory geomechanical tests on core from the area and full wave sonic logging information was used to generate a model which calculates the critical drawdown for the problematic sands. Critical drawdown control has been the solution for most wells. Some other solutions such as hydraulic fracture and highly deviated wells in the correct direction have also been utilized.

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.