Downhole pH Measurement for WBM Contamination Monitoring and Transition Zone Characterization

Abstract

Abstract Formation water sampling is important for understanding corrosion and scaling potential of the water, characterizing hydrocarbon/water transition zones, determining compatibility between injection water and formation water, and determining optimal salinity and pH windows for polymer and gel injections. To obtain good quality formation water samples with wireline formation testers in wells drilled with water-base mud (WBM), it is important to track mud filtrate contamination by distinguishing between formation water and mud filtrate in real time while sampling. Current techniques typically use a resistivity sensor to monitor this contamination in real time. However, this works only when there is sufficient resistivity contrast between the filtrate and the formation water. Another technique involves addition of a colored tracer dye to the drilling mud and then monitoring the tracer concentration with an optical detector in real time. In this paper, we present results from a Middle East production well in which a new downhole pH measurement sensor was deployed on a formation wireline tester. The sensor uses pH-sensitive dyes that change color according to the pH of the water sample. The pH of the formation water is a key water chemistry parameter that can alter because of temperature and pressure changes surrounding the sample as it is brought to surface. Hence, to obtain accurate values, pH must be measured in real time at downhole reservoir conditions. In addition to successfully measuring pH downhole, results from this field test show that pH can also be used to qualitatively monitor mud filtrate contamination in real time. This takes advantage of the fact that the mud filtrate pH is typically much higher than the pH of formation waters. Multiple pH measurements during sampling showed a good match with cleanup trends determined independently using a colored tracer added to the mud. Simulations using some typical formation water and mud filtrate samples indicate that the pH dependence on contamination is a complex function depending on several factors including composition, buffer capacity, ion equilibria, ionic strength, temperature, and pressure. This makes quantitative estimation of contamination from pH measurements difficult; nevertheless, qualitative tracking of cleanup trends is possible and useful. The ability to differentiate between formation water and mud filtrate pH is also useful in characterizing oil/water transition zones and delineating oil/water contacts by scanning formation fluid samples at different depths. Both of these downhole pH measurement applications improve the efficiency and success rate of sampling with wireline formation testers.