Identifying Potential Water Production in Tight Gas, Shaly-Sand Reservoirs Using Magnetic Resonance Imaging Logs: A Case Study from South Texas

Abstract

Proposal Complexities in the properties of tight gas sandstone formations, due to low effective porosities, presence of clays, the effect of mixed mineralogy, etc. require advanced methods of evaluation. Likewise, adverse effects brought on by drilling, (e.g., altered formation, unknown mud filtrate effects) must be taken into account. Additionally, variations in formation water salinity "Rw" value, being a primary input to log analysis, must be determined when using conventional log interpretation. Accordingly, in tight gas sands, operators usually resort to well testing for characterization of tight gas sands. Effective well log evaluation will offer a first approximation as to the selections of best zones to be tested. Magnetic Resonance Imaging "MRI" technology contributes significantly in the evaluation of complex tight shaly-sands, because it offers in-situ evaluation helpful in determining fluid producibility and collaborates the conventional resistivity based log interpretation. All resistivity based log analyses require formation water salinity "Rw" information. However, MRI logs offer a "non-resistivity" based fluid identification and "stand alone" interpretation. In the case of tight gas sands, where effective porosities are low, sensitivity of log interpretation to Rw becomes more important. This paper illustrates such a case and offers the MRI as a mean to circumvent the issue of "Rw" determination as well as to verify porosity and provide a permeability indicator. Such an indicator can be calibrated when integrated with the results from core analysis. The ability of MRI to detect the type of fluid in the formation is intended to influence selection of zones to be tested in an exploration well. This article demonstrates the application of the MRI logs in characterizing tight gas shaly-sand formations in a South Texas exploration well where the "Rw" from produced water samples was different from the assumed "Rw" from an analogous field. It will also argue for the value of MRI technology in minimizing the cost and associated risk of well testing. This risk is minimized by using the MRI log to provide information on the fluid type (water or gas) present in each zone. Introduction Economics of testing and producing tight gas shaly sands is the impetus for exploring the value of the MRI service. In tight shaly sand formations, merely determining producible gas is not sufficient to guarantee an economic outcome. Key factors for evaluating this type of formation are, determining adequate net interval(s), and fluid typing (gas vs. moveable water) primarily to maximize producible hydrocarbon, and avoid the possibility of producible moveable water. All resistivity based log analyses require formation water salinity or "Rw" information. In the absence of accurate "Rw" information, the MRI offers a "non-resistivity" based fluid identification and "stand alone" interpretation. Log calculations using resistivity and conventional triple combo porosities in formations having capillary bound water in very small pore throats and/or high clay content may lead to large water saturation values. This creates the vulnerability for overestimation or underestimation of formation water saturation. Therefore, amongst other considerations, accurate formation water saturation determination and hence, water salinity "Rw" values are imperative inputs. Unknown or questionable formation water salinity "Rw" values can cause net pay calculations too be to high or too low. As a consequence, operators may resort to the costly and time consuming process of well testing. Establishing effective log evaluation methods for tight gas sands is the preferable and most economic technique to minimize testing time and cost. Magnetic Resonance Imaging "MRI" technology contributes significantly in providing effective and economic ways for evaluating tight gas shaly sands especially in exploration wells1–8. This paper discusses the MRI as a means to circumvent the "Rw" problem as well as to verify free-fluid porosity and provide a reliable permeability indicator. The fluid typing ability of MRI logs assists in selection of high potential gas zones to optimize the completion.