Sabiriyah Mauddud, has been on water flood since Dec 2000 with seawater. Reservoir souring is the in-situ generation of H2S in the reservoir itself due to waterflood operations using seawater. Traditionally, Mauddud is a sweet reservoir & some degree of reservoir souring occurs due to Sulfate-Reducing Bacterial (SRB) activity. As a new initiative for tracking souring, H2S coupons were used for the first time in Kuwait at one of the EOR wells in open hole condition during drilling. This has been extended to flowing wells as part of the FBHP survey so that the coupons reflect H2S levels nearest to Mauddud perforations, using the slick line, for the first time in the industry, to detect micro H2S levels.
Different methods are used to measure H2S concentrations such as WH samples or titration / dragger tubes from needle valve (separator unit) for surface sampling, or taking bottom hole sample using conventional sampling chamber which is usually made from stainless steel material, normally reactive with H2S.
It is a challenge to accurately measure low ppm levels of H2S in formation fluids where there is a risk of not capturing these concentrations using conventional samplers/ sample bottles because of the reaction between H2S and sample chamber during bottom hole sampling operation. Therefore, non-reactive coated sampling bottles were recommended and used in the past. The option of H2S coupons to be used as slick line operation is one of the key improvements in tracking H2S without loss of tiny content of H2S via adsorption in wellbore/ flowline transit.
With H2S coupons, certain metal alloys discolor, as a result of corrosion, only due H2S. These alloys are said to be "Specific" to the presence of H2S. Discoloration due to H2S is weakly dependent on temperature and exposure time. Discoloration results in loss of the original shiny or glossy finish. This phenomenon is largely a function of partial pressure, not concentration. This is true for both gas phase and liquid phase. Hence, the H2S coupons measures the total H2S for the reservoir fluid.
SA-X is a vertical well in Sabiriyah Mauddud equipped with Y-Tool. H2S coupons were run to detect H2S levels across Mauddud. Two sets of H2S coupons were placed inside the bottom of the tool along with pressure and temperature sensors. Consistent results were obtained from both H2S coupons sets. Only the high sensitive coupon was affected indicating H2S partial pressure higher than 0.005 but less than 0.018. H2S level is found in the range between 5 – 15 ppm which is quite close to what has been measured in the lab during compositional analysis during PVT studies for monophasic fluid. This has built team's confidence regarding the validity of the data via souring coupons. This procedure has now become part & parcel of our routine surveillance activities for tracking reservoir souring.
The procedure has added a simple but accurate fit-to-purpose tool for tracking reservoir souring in North Kuwait.