In this paper a new method of performing a wireline large scale formation test is introduced. To measure reservoir properties a significant distance from the wellbore, drill stem testing (DST) tests are often employed. In addition to measuring reservoir dynamic properties, a DST provides information regarding reservoir geometry including the extent of the reservoir, upper and or lower boundaries of the reservoir, barriers to flow within the reservoir. The DST is considered the gold standard for dynamic parameters such as reservoir pressure, formation mobility/permeability, well bore skin factor, flow anisotropy, and the best estimate of production potential for the measured interval. Conceptually, a typical DST is simply a controlled limited production of formation fluids over the reservoir interval of interest. However, a full DST has become an operationally intensive and costly endeavor often lasting many days to weeks. Planning and executing a DST is costly requiring highly specialized equipment that is customized for a unique set of reservoir and operating conditions. One of the primary considerations is to insure the environmental compliant disposal of a considerable quantity of produced fluid.
Due to the high cost of traditional DSTs, alternate methods of testing have been employed. Wireline formation testers capabilities have increased their pumping capacities and are used to perform a small scale mini-DST that has increasingly been employed as a substitute for full DSTs at a much lower cost. A mini-DST can provide measurements of the reservoir dynamic properties, albeit over a smaller depth interval and smaller radius of investigation. The mini-DST radius of investigation generally extends up to 100feet from the wellbore and over a vertical height of a few feet as opposed to the full DST which can extend thousands of feet from the wellbore and be extended over tens of feet covering an entire vertical producing interval. While a DST can accurately characterize an entire formation production interval's potential, the mini-DST can delineate the formation flow interval and determine the most productive layers, flow barriers and thief zones which are critical to optimizing the completion design.
This new method combines a conventional wireline mini-DST and sampling procedure with an extended injection test in order to recover the reservoir flow interval geometry information and improve flow interval delineation. The mini-DST measures the local mobility of the formation, cleans the wellbore of mud filter cake and near well bore mud filtrate contamination, thus enabling the acquisition of a clean formation sample. When acquiring a formation sample the formation fluid properties can be determined with downhole sensors and a subsequent mini-DST can provide the in situ dynamic properties. A selected injection fluid can be used to reverse the process by flowing into the interval. Because the injection fluid would have known properties that are measured in controlled laboratory testing, the dynamic data results will be more definitive. For example, the viscosity will be known enabling the rock permeability determination, where traditionally only the mobility can be determined. In many cases the reservoir oil type is known and the injection fluid can be closely matched to improve the in situ dynamic data results. Aided by favorable backing pressure from the surface, and not limited by the fluid bubble point, the injection fluid may be pumped into the formation at a higher rate than the formation fluid can be withdrawn from the formation. This allows a greater sand face to reservoir pressure differential, yielding an improved pressure signal for evaluation. Furthermore, because fluid is injected, no environmental sensitive disposal is required, and the procedure is inherently safer for maintaining well integrity than a formation fluid withdraw. However, interpretation of the pressure rebound after the injection stops may require a reservoir simulation if the injection fluid property is significantly different from the in situ reservoir mobile fluid.
This paper evaluates the new method based on detailed reservoir simulations and using available field equipment capabilities. These simulations include considerations of invasion and cleanup in a multiphase environment. In addition, a sensitivity study summarizes the testing effectiveness by varying the primary parameters such as permeability, anisotropy, skin, and formation barrier distances. From this work, conclusions are drawn comparing the new reverse injection DST method to traditional DST and wireline technologies.