This paper demonstrates the connection between laboratory measurements and field results from gelant treatments in production wells at the naturally fractured Motatan field in Venezuela. Using a HPAM polymer with an organic crosslinker, laboratory corefloods revealed that under reservoir conditions, the gel provided oil and water residual resistance factors of 20 and 200, respectively. This gel was placed in several production wells in the Motatan field. In Well P-47, 1,000 bbl of this gel reduced the water cut from 97% to 64% and increased the oil production rate by 36%. The success of these treatments depends on the distance of gelant leakoff from the fracture face and the in situ residual resistance factors in the oil and water zones. Analyses were performed to determine these parameters, based on formation permeabilities, porosities, fluid saturations, fluid properties, fluid production rates, and pressure drops before, during, and after gelant placement. Accurate pressure drops before, during, and after gelant placement were particularly important. Sensitivity studies were performed to demonstrate their significance and the impact of measurement errors. A methodology is presented for optimizing the volume of gelant injected for these applications.
Introduction
For most gel treatments applied for conformance improvement and water shutoff, design procedures (especially the methods for treatment sizing) were strictly empirical - a fact that is partly responsible for the erratic success rates of these treatments. Many water shutoff treatments rely on the ability of polymers or gels to reduce permeability to water much more than that to oil. Unfortunately, the magnitude of this disproportionate permeability reduction cannot yet be predicted a priori under reservoir conditions. Since laboratory studies are rarely performed before field applications, widely varying field results are not surprising.
In some cases, individuals have suggested that field results with gelant treatments were at odds with laboratory data or with basic petroleum engineering principles. Depending on their background, operators, service companies, and researchers naturally place more credence in some observations than others. For example, a service company may prefer to emphasize certain field observations to rationalize an explanation that researchers find in contradiction with laboratory findings or in violation of established petroleum engineering principles. Consequently, all data (field, laboratory, and theoretical) should be considered when applying and evaluating field applications of gelant treatments. Of course, observations can be misinterpreted. Laboratory experiments may be botched or performed in misleading ways; theoretical or numerical studies may suffer from incorrect assumptions (e.g., garbage in/garbage out); and field results may be interpreted incorrectly. However, by combining sound laboratory, theoretical, and field observations, a consistent picture should emerge that can be used to improve the success rate for future field applications.
