Development and Use of Improved Wireline Formation Tester Technologies in Challenging Deltaic Reservoirs

Abstract

Many sedimentary features of gas fields are multilayered, deltaic, thinly laminated shaly sandstones consisting of channel and bar sands with limited lateral and vertical extension. Relying only on conventional openhole log data and performing correlations among nearby wells proved to be inconclusive in identifying gas reservoirs owing to their thin beds, high shale content, and variable formation water resistivity. Missing gas-bearing formations translates into lost productivity, while perforating water zones can have detrimental effects on well performance. Moreover, the limited lateral extent of these relatively tight gas sands leads to extremely depleted reservoirs alternating with layers with virgin zone pressures. As a consequence, the depleted layers face a significant overbalance while drilling with an oil-base mud system. Given these complexities, fluid identification and pressure measurements have a significant impact in resolving key uncertainties of such reservoirs. The main challenges faced during formation testing in the reservoirs studied have been a) laminated, low mobility and thin formations with varying water salinity, b) high depletion, resulting in extreme overbalance for some layers in new wells, c) possible formation damage while drilling, d) cable creep while station logging. Several different approaches have been recently launched to increase the success ratio of wireline formation testers (WFT's) in getting reliable pressures and fluid analysis, including real-time monitoring of each survey by reservoir engineers. This paper describes the development path and results from the new techniques:extra-large diameter probe,elliptical probe,the openhole driller,cable creep correction andextra-extra high displacement pump unit. We will present each project and its impact on the improvement of WFT tester success ratio in such challenging environments. Introduction The predominant sedimentary features in the reservoirs we focus are very thinly laminated shaly sands composed of 70–80% quartz plus feldspar and clays (kaolinite and illite), in which gas sands are not in pressure communication. Figure 1 shows an image log of a 4-meter section where very fine layering is evident. Vertical heterogeneity on various scales lead to multiple gas/water contacts with extremely depleted and virgin zones in the same well, thus resulting in very high over-balance; commonly in excess of 6000 psi and occasionally up to 10000 psi differential pressure. The shale quantity and thin-beds very often results in conventional logs giving wrong fluid determination, therefore fluid analysis using wireline formation testers is a very important step during the open hole evaluation stage. As noted, these conditions are quite challenging for formation testing. Some of these challenges, particularly near wellbore formation alteration have been studied using a multi-probe wireline formation tester (Ayan et al., 2007). In this study, the authors used dipole radial profiling and Interval Pressure Transient Tests (IPTT) and showed that possible formation damage does not necessarily increase with increasing overbalance. Some operational aspects of wireline formation testing have been discussed for such environments (Ferment et al. 2004), highlighting issues with high differentials, probe plugging, fine laminations and depth control. Over the past 20 years, in the formations we focus in this study, the operational success ratio of downhole formation pressure testing (valid test vs. total tests) has remained at an average of 30% despite technological innovations in both wireline and drilling. In Table 1, we summarize the main reasons causing low pretest success ratio. To increase the success ratio for pressure testing and downhole fluid identification, a multitude of solutions were proposed. In this study, we describe each of them and the results achieved following their introduction.

Single-well In-situ Measurement of Residual Oil Saturation after an EOR Chemical Flood

“…The probe failed to inject because of mud cake and the dual packer produced uncontrolled injection geometry. Injection was successful through a small drilled hole (Orban et al 2009). The drilling tool includes a 10mm bit on a flexible shaft exiting a rubber pad that isolates the drilled hole from the mud column.…”