In development of mature oil fields using enhanced oil recovery (EOR) techniques, one of the challenges is to quantify remaining oil and to evaluate the potential gain of EOR in pilot studies. One of the proven technologies to estimate remaining amounts of oil is the single-well chemical tracer test (SWCTT). During such push-and-pull tests, oil/water partitioning ester partially hydrolyses to a non-partitioning water tracer. A time-lag in back-production time between the injected ester and the alcohol generated in-situ yields remaining oil saturation (ROS) through a simple relation. A similar time-lag technique is used in partitioning inter-well tracer tests (PITTs), where tracers are injected into injectors and sampled in producers. New and stable, oil-water partitioning tracers suitable for oil reservoir PITTs have been recently developed and field tested (SPE164059), allowing measurement of remaining oil saturation in inter-well regions. This paper reviews methodology to assess oil saturation in both near-well and inter-well regions of an oil reservoir, and highlights the differences and similarities of partitioning near-well and inter-well tracer tests that can be used to evaluate the potential gain from an EOR-operation. SWCTTs and PITTs target different scales of an oil field, as a SWCTT explores the near well zone, up to a few meters, and a PITT explore an inter-well region. In order to assess information on both these scales we propose a systematic procedure for oil saturation measurement, using partitioning tracers. The procedure involves use of PITTs and systematic residence time distribution (RTD) interpretation of tracer production curves to extract information about remaining oil saturation and the distribution of this saturation in an oil field. The procedure is validated using the tracer data recently reported by Viig et al. (SPE164059).
Tracers are increasingly being used as effective reservoir monitoring and surveillance (M&S) tool in the oil and gas industry. They can qualitatively or quantitatively gauge how fluid flows through the reservoir. Tracer surveys, conducted either as interwell tests or single-well tests, are one of the enabling technologies that can be deployed to investigate reservoir flow performance, reservoir connectivity, residual oil saturation and reservoir properties that control displacement processes, particularly in improved oil recovery (IOR) and enhanced oil recovery (EOR) operations. An inter-well chemical tracer test (IWCTT) was designed and implemented in a carbonate field with a diffuse fracture network on peripheral water-flood to determine reservoir connectivity, water breakthrough times, and fluid saturations. The reservoir is heterogeneous with layers of high to low permeability limestone with porosity a range of 20-30%, and interspersed with patchy layers of dolomites and a very competent anhydrite seal. The IWCTT was conducted on a very mature part of the field near the peripheral water injectors to determine the effectiveness of infill injector wells. Four distinct tracers were injected into four individual injectors and the residuals were monitored in four “paired” producers. This paper reviews the complete design and implementation of the test, operational issues, and the analyses and interpretation of the results. The breakthrough times of the tracers are reported and interwell connectivity between the paired and cross-paired injectors and producers are analyzed. Tracers were detected in three of the paired producers, albeit at different breakthrough times. One tracer was also detected in a cross-paired producer. The tracer results were modeled using the current reservoir simulation model with fairly good matches. The tracer data were then used to fine tune the current reservoir simulation model. The IWCTT has been very effective in analyzing well interconnectivity in the reservoir and has led to better reservoir description and an improved dynamic simulation model. Plans for further implementation of this important M&S tool will also be shared.
The success of any improved oil recovery (IOR) project is largely dependent on how much oil is remaining to be mobilized within the targeted area of the partially depleted or mature reservoir. Partitioning tracers are generally used to measure residual oil saturation (Sor) or remaining oil saturation (ROS) in the near wellbore region via a single well chemical tracer test (SWCTT) or in an inter-well region via a partitioning inter-well tracer test (PITT). There is a limited repertoire of nonradioactive and environmentally friendly inter-well partitioning tracers for measuring ROS. A new class of environmentally friendly partitioning tracers was field tested, in a giant carbonate reservoir undergoing peripheral waterflood, for measuring ROS in inter-well regions in a depleted area. The new partitioning tracers were qualified via laboratory experiments and are deemed to be very stable at reservoir conditions (213°F and a salinity range of 60-200 kppm). The field pilot was conducted concurrently with a set of non-partitioning inter-well chemical tracer test (IWCTT) to determine reservoir connectivity, water breakthrough times, and injector-to-producer pair communication in an area selected for an IOR/EOR field pilot. An elaborate sampling and analysis program was carried out over a period of 30 months. This paper reviews the complete design and implementation of the test, operational issues, and the analyses and interpretation of the results. The breakthrough times of the passive and partitioning tracers are reported, and inter-well connectivity between the paired and cross-paired injectors and producers are analyzed. The ROS measured by a majority of the novel tracers is comparable to the saturations obtained via SWCTT, core and log derived saturations. The combination of conventional IWCTT and the novel partitioning tracers via PITT has been very useful in analyzing well interconnectivity, understanding the reservoir dynamics and quantifying remaining oil saturation distribution in the reservoir. This has led to better reservoir description and an improved dynamic simulation model.
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.
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