Real-Time Determination of Filtrate Contamination During Openhole Wireline Sampling by Optical Spectroscopy

Abstract

Abstract The key objectives of openhole wireline sampling are to identify desiredfluids and to collect representative formation samples. Optical methods such asvisible - near-infrared spectroscopy (NIR) are used to identify and quantifymixed oil, water, and gas flows in-situ during wireline logging. In wellsdrilled with oil based muds (OBM) or synthetic based muds (SBM), differentiating crude oil from the filtrate base fluid is critical. The widely used current practice is to measure the increase in fluid coloration (by NIR)vs. time to qualitatively assess the increasing fraction of crude oil with pumping time. Here, we describe new methods of utilizing NIR log data toprovide quantitatively during sampling the %-OBM contamination in the extracted fluid. In addition, these methods allow prediction of contamination at futuretimes during the sampling job, thereby determining the effectiveness of further pumping. The significant improvement in sampling efficiency is demonstratedwith several log examples. Finally, using our methods, mud performance in the downhole environment can be monitored and optimized in subsequent jobs. Introduction Hydrocarbon properties vary over a huge range, from dry natural gas toviscous tars at the extremes. Hydrocarbon components such as wax, asphaltenes, and gas as well as their phase behavior have an inordinate impact on productionstrategies. Open hole sampling is an effective way to acquire representative reservoir fluids. Sample acquisition allows determination of criticalinformation for assessing the economic value of reserves. In addition, optimal production strategies can be designed to handle these complex fluids. Inopenhole sampling, initially, the flow from the formation contains considerable filtrate, but as this filtrate is drained, the flow increasingly becomes richerin formation fluid.1,2 Sampling in wells drilled with water based muds is efficiently performed. NIR spectroscopy can readily distinguish watervs oil, thus indicating when sampling should take place; 2 small quantities of water are easily separated from crude oil in the lab providing representative formation crude oil samples. In addition, gas detection usingoptics determines that sampling is performed at pressures higher than bubble point assuring representative sampling.3 Sampling in wells drilled with OBM and SBM represents a particular challenge. (For our purpose, SBM and OBM behave similarly, so we will referonly to OBM but SBM's apply also.) The OBM (and SBM) filtrate is miscible withcrude oil, thus cannot be separated. Greater than 10% contamination of thecrude oil sample by OBM filtrate makes it difficult to determine virgin crudeoil properties.1 In addition, proper extrapolation to virgin crudeoil properties from measured properties of contaminated samples requires accurate determination of contamination. Contamination estimation is difficult to perform as evidenced by common discrepancies among different lab analyses. It is best to reduce the contamination to low levels during sampling, yieldingclean samples requiring little parameter extrapolation. However, it is notacceptable to pump indefinitely to reduce contamination levels because rig timefor OBM wells is often very expensive and the chances of tool sticking can onlyget worse with lengthy pumping times. Also, in some cases, contamination levelsremain stubbornly high even after extended pumping.