Multidisciplinary Approach and New Technology Improves Carbonate Reservoir Evaluation
Authors: Okuyiga Moyosore O. , Berrim Ahmed , Haddad Sammy S. , Xian ChengGang , Abu Lawi Majed Shaaban 
Published: February 2016

Wireline formation testing in low permeability carbonate reservoirs of the UAE has been challenging with frequent tool plugging, extended pumping time, and multiple trips out of the hole. At the same time, there are increasing demands of various types of formation testing measurements to satisfy various reservoir evaluation objectives. Thus the complexity of the wireline formation testing (WFT) has dramatically risen and continues to rise. It requires an effective solution to significantly reduce the largely extended rig time due to heavy WFT programs and operation issues, and at the same time, maintain data quality and achieve even better data quality to fully satisfy planned objectives. A recent well (Well B in this paper), drilled offshore Abu Dhabi for ADMA-OPCO, penetrated the main three reservoirs and presented a rare opportunity to address these challenges and implement a newly proposed solution.

The tested intervals contain multiple thin zones, which make it difficult to predict reservoir fluid types from openhole logs or pressure gradients alone. An extensive Downhole Fluid Analysis (DFA) program was needed for fluid types and contacts determination. A multidisciplinary team between the operator and the service company was organized to carefully consider of the challenges mentioned above, identify the source problems and recommend a solution. The solution involved the economically viable replacement of the drilling fluid with a low solid content fluid prior to the wireline formation testing operations. A new wireline formation tester technology, the Extra Large Diameter module (XLD), was also recommended to replace the dual-packer at a broad permeability range. The combination of drilling fluid displacement, XLD utilization and the two-pump configuration proved to be major operational factors in reducing the overall job duration, rig time, and smoothly achieving the required objectives.

As the results, a record job including 27 DFA stations (16 with samples), 2 mini-frac stations and in access of 91 pressure points was achieved. The DFA and samples provided significant answers and clues to a broad range of reservoir issues including compartmentalization, fluid compositional variation, the unexpected identification of oil, oil reserve assessment, and gas compositional analysis to understand its origin and potential contact migration. The wireline formation testing results have added direct values to reservoir evaluation and development. In this paper, a best operating practice for similar reservoirs will be presented.

Introduction

The evolution of wireline formation testing technologies including new techniques, applications and tool reliabilities over the past few years has caused a significant change in perspective with regard to understanding reservoir rocks and fluids and their heterogeneities. The most significantly innovative development of formation testing technologies is Downhole Fluid Analysis (DFA), which is rapidly advancing and the list of its measurements is expanding commensurately. Not only DFA characterize fluid physical and chemical properties, also it is being increasingly applied for formation evaluation. Nowadays, using WFT, it is highly possible to simultaneously characterize extreme rock and fluid complexities with sufficiently high resolutions so as to greatly aid in reservoir evaluations such as identification of compartmentalization, fluid spatial variations, characterization of hydrocarbon-water transition zones, injection fluid monitoring, and so on.

On the other hand, the complexity of the WFT operation has dramatically risen and continues to rise, due to the increase in the types of WFT modules being run, and more DFA stations more DFA measurements, and more targeted reservoir zones. One of big concerns is how to significantly reduce the largely extended rig time due to such complexity of WFT program, and at the same time, maintain data quality, and achieve even better data quality.