Single Well Chemical Tracer Tests to Assess Low Salinity Water and Surfactant EOR Processes in West Africa

Abstract

Abstract Assessing the residual oil saturation of different EOR techniques is of paramount importance to accurately select a suitable tertiary development and properly quantify remaining reserves in a given field. The Single Well Chemical Tracer Test (SWCTT) has proved to be a reliable technique to consistently measure the residual oil saturation in a representative formation volume. This work describes the design, operations and results of a sequence of three SWCTTs performed in a very heterogeneous and complex reservoir, on-shore in West Africa. The aim of the pilot tests is the evaluation of low salinity water and surfactant flood efficiency compared to current sea water injection. The EOR techniques were selected to improve the oil recovery factor in the field after an intensive experimental work and the effects were evaluated by means of simulation models. The SWCTTs were executed with cycles of injection, shut-in and production periods to measure the residual oil saturation after sea water, low salinity water and then surfactant injection to compare with experimental and simulation results. Favorable results were achieved for surfactant flooding with considerable reduction in residual oil, confirming that a performing surfactant mixture at harsh field condition was found. The SWCTT results showed, instead, a minor effect for low salinity water and the difference in core and field results were then investigated. A peculiar SWCTT program was executed to verify and compare the efficiency of two EOR methods at field conditions. Thanks to the encouraging surfactant results and the gained field experience, an extension of the EOR technique is under design in a cost-effective surfactant-polymer inter-well pilot. This assessment will provide optimization of chemical formulation in terms of compatibility, surfactant concentration, polymer viscosity and slug volumes. Introduction A key technical input parameter for any EOR project is the assessment of residual oil saturation (Sor) after primary and secondary development. To establish what is left in reservoir is fundamental to assess the benefits for any further development. The Sor and its distribution in the field is a critical parameter. Several methods are available to estimate Sor at different resolution scale, ranging from analogues, lab measurements, logging techniques, Single Well Chemical Tracer Test (SWCTT) and Partitioning Inter-well Tracer Test (PITT) ([3], [5]). The SWCTT is a field trial that provides, using tracer technology, a consistent estimate of Sor at a near-wellbore scale resolution, thus in a representative area of formation (typical radius of investigation is 2–6 m). This test has become a popular technology among oil companies to directly measure Sor and to evaluate rapidly EOR processes using a single production well. It represents an intermediate step from laboratory work and simulation EOR studies to full field application.