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In certain situations, it is necessary to obtain a reliable measurement for connate water saturation (Swc) in an oil reservoir. The single well chemical tracer (SWCT) method has been used successfully for this purpose. The SWCT method has been used successfully for this purpose in six reservoirs. The SWCT test for Swc usually is carried out on wells that are essentially 100% oil producers. The procedure is analogous to the SWCT method for Sor, taking into account that oil is the mobile phase and water is stationary in the pore space.
Abstract The streamline-based tracer model has been successfully deployed to history match and predict miscible and immiscible Water Alternating Gas (WAG) processes at the field scale. The tracer model is a simplified method for the three-phase WAG process, and is computed parallel to the traditional streamline waterflood model. This paper provides an overview to illustrate the relevant concepts and applications of the streamline-based tracer model. A Prudhoe Bay example of the vertical Miscible Injection Stimulation Technique (MIST) is presented to demonstrate and to verify the field use of the tracer model. Introduction WAG injection has been recognized as an effective improved oil recovery (IOR) procedure and is widely applied to enhance trapped oil production in reservoirs such as in Prudhoe Bay. Our knowledge of the controlling physics of WAG injection in the field can be limited. WAG injection is a complex multiphase process influenced by important factors such as geologic heterogeneity, gravity, phase interactions resulting in changes in mobility, among many others. Historical efforts to develop simulation tools for WAG processes to history match and to predict field scale IOR operations have proven to be difficult yet challenging. Accurate modeling requires fine-scale, three-dimensional, fully compositional models that simulate rapid gas movements in a reservoir containing possibly thousands of wells. Such models can be very CPU-intensive for real reservoir management and decision-making. With the continuing development of streamline technology, it is possible to construct a field-scale, 3D, compositional, three-phase streamline model to simulate the dominant physics of the WAG process, although CPU times can still be large. The streamline model used in this paper was designed when the streamline simulator could only support two-phases. The tracer model is therefore an add-on to a traditional two-phase (oil and water), front-tracking reservoir simulator, leading to speed-up and flexibility in simulating the WAG IOR process. Future enhancements to the tracer model will utilize more of the current simulator's 3D and three-phase capabilities. This tracer add-on model was designed to simulate the WAG injection process in a simplified two-phase setting. Highly complex, faulted grids can be modeled and large times step can be taken with minimal numerical dispersion. As wellrates change, front locations are mapped and propagated along updated streamlines. This approach takes advantage of minimal numerical dispersion, which plays havoc with finite-difference prediction alternatives. This leads to the possibility of running large fine-scale models very quickly. The tracer model consists of two independent displacement processes propagating in parallel along each streamline. The first process is the traditional waterflood to reduce oil saturation to waterflood residual level; the second process is the MWAG to further reduce oil saturation to below waterflood residual level. An important feature of the streamline-based tracer model is the explicit modeling of IOR oil (i.e., the incremental oil recovery over waterflood) spatial distribution. In this paper, we use the term IOR to strictly refer to the incremental oil from the WAG injection operation in the field. The IOR displacement process is mimicked by the simplified movement of solvent and IOR tracers, each tracer flows along the existing streamline (for oil and water) at a user-specified multiple (accessible pore volume factor) of the Darcy velocity. This explicit approach allows streamlines to reveal WAG-injector/IOR-producer pairs and clusters dynamically.
Cockin, A.P. (BP Exploration Co. Ltd.) | Malcolm, L.T. (BP Exploration Co. Ltd.) | McGuire, P.L. (Arco Alaska Inc.) | Giordano, R.M. (Arco Exploration & Production Technology) | Sitz, C.D. (Chemical Tracers, Inc.)
Abstract In 1990 a single well chemical tracer (SWCT) test was performed in Prudhoe Bay to measure the effective water flood and miscible gas flood residuals over a 12 ft reservoir interval. This is believed to be the first such use of this technology for a hydrocarbon miscible gas. This paper describes how the usual SWCT design was modified to accommodate the miscible gas, the results of the SWCT, which for the miscible gas part were significantly higher than miscible gas coreflood residuals, and the subsequent simulation of the test which has provided good agreement with the observed results. The paper explains, with simulation support, what caused the measured residuals to be higher than expected, and draws on the experiences of this test to make recommendations for the design of future SWCT tests measuring residuals to gas flooding. P. 61
Summary The single-well chemical tracer (SWCT) method was used to measure resident water saturation in two wells in the Ivishak reservoir of the Prudhoe Bay field. The tests, performed in 1981, were in API Wells 406 and 488, both of which had been oil-base cored through the Ivishak. The results of the tracer test in Well API 406 were interpreted by conventional SWCT simulation procedures. A three-layer simulator model was used to match the tracer production profiles measured during the test. The average water saturation obtained from the simulator match was 16±3% PV, in good agreement with the Dean-Stark analysis of the oil-base cores. The tracer-test results from Well API 488 were more difficult to interpret because of the very low water saturation at this location in the Ivishak. The SWCT method requires a hydrolysis reaction to take place in the formation to produce a product tracer. In this case, the amount of water present was so small that very little product tracer was formed. The interpreted Sw for this well was 2±2% PV, again in reasonable agreement with the oil-base-core measured value of 3.8%. Sw Determinations in the Ivishak Introduction. The estimation of hydrocarbon pore volume, VpHC in the Ivishak (commonly called the Sadlerochit) reservoir of the Prudhoe Bay field used the general relationship given in Eq. 1.Equation 1 Because Sw was suspected to vary considerably within the oil zone, accurate estimation of VpHC demanded good measurements of Sw at a reasonable number of locations in the reservoir. For reasons discussed by McCoy and Grieves, electric log accuracy was inadequate in some parts of the Sadlerochit. The primary method chosen to measure Sw was Dean-Stark analysis of samples from cores taken with oil-base drilling fluids. As an independent confirmation, in-situ measurements of Sw in two of the oil-base-cored wells by use of the SWCT method were performed. Tracer Methods for Measuring S w . The SWCT method was originally developed by Deans and Deans et al. to measure residual oil saturation, Sor, after waterflooding. Since 1968, this technique has been used in more than 200 sandstone and carbonate formations. Well-to-well tracer methods that use the same principles have been less frequently reported. Operational considerations make a single-well procedure much more practical. As practiced, the SWCT method measures an average residual saturation over a relatively large volume of pore space. Typical depth of investigation is 10 to 20 ft away from the wellbore, which should be beyond the region of alteration caused by drilling and completion operations.