Hydrocarbon Potential Monitoring in Gas Sandstone Reservoir Using CHFR and TDT Techniques

Abstract

Abstract The ability to detect and evaluate bypassed hydrocarbon and track fluid movement in sandstone reservoir is vital in the quest to improve production and increases recovery. The main technique, which has been used for monitoring reservoir saturations, is Thermal Decay Time (TDT) tool. But, it was hard to interpret the TDT data in the low formation water salinity reservoirs. This problem can not be solved because it is related to the theory itself of TDT measurement, which depends on salt content in formation brine. The problem of low formation water salinity was solved by Carbon Oxygen Ratio (COR) technique, which was latterly combined with TDT technique in the same tool, called reservoir Saturation Tool (RST) tool. This tool was not widely used due to its short depth of investigation (6–8 inches) in addition to its long logging time. A Technique called Cased Hole Formation Resistivity tool (CHFR) technique was proposed to overcome many of the pulsed neutrons tool limitations. Based on the actual CHFR logs recorded through wells studied in an oil field in Sinai, which are presented and discussed in the present work, a comparison study was done between the two available techniques, TDT and CHFR as methods for reservoir saturation monitoring, in addition to the results of open-hole resistivity logs as reference runs. It was found that water saturations calculated from CHFR logs were more accurate than TDT log in most of the cases, and that the quick look of CHFR logs always agrees with its quantitative interpretation, which gives the trust to relay on quick look of CHFR log, if a fast decision is required. While the quick look of TDT log was found to be very tricky, in most of the cases: often does not agree with its quantitative interpretation. Introduction Continuous reservoir saturation monitoring, through cased wells, usually is the main key for proper reservoir management and recovery optimization especially for huge and mature oil fields. The main technique, which has been used for monitoring reservoir saturations, is TDT tool. One of the main problems that encountered while using TDT log is the reservoirs with low formation water salinity. This problem was very obvious in some wells in studied oil fields located in Sinai and producing from sandstone formations. These formations have naturally low salinity formation water (about 20,000 -30,000 ppm). Also this problem could be appeared in reservoirs that are supported by water injection projects, in which the formation water is diluted by the injected water. This is because the injected water, used in water injection project in this field, is the Red sea water. The average salinity of this water is about 40,000 ppm, which is greatly less than average salinity of formation water for other zones (about 150,000 ppm). The problem concerning the low formation water salinity is actually related to the theory of TDT itself because its measurements depend upon the Chlorine (NaCl) content in the formation water, which is highly dependent on the salinity of formation water. So, TDT, in many cases, can't distinguish between the low salinity water (salinity below 60,000 ppm.) and oil. Therefore, wells drilled in reservoirs of low-salinity water could not be evaluated by TDT log at all. This problem has been solved by the Carbon-Oxygen (C/O) technique, which was combined with TDT technique into another tool called RST tool, [1–3]. Some other problems were being faced while using TDT log: the wells with high pressures or wells which need to be killed before workover. Also wells that completed with Electrical Submersible Pump (ESP) artificial lift strings (about 88 % of wells in the selected field are completed with (ESP). TDT tool can not be run through this production string to record against producing formation because the string is close ended by the ESP. therefore, in order to record the TDT log, production string must be pulled out of hole first, which needs killing the well by a killing fluid. The actual problem, encountered in the above mentioned cases, is that the killing fluids may invade the producing zone and this invaded zone will affect greatly the results of TDT logs. This problem may also be encountered in wells which are producing with high water cut and shut-off for long time. The segregated water will invade the hydrocarbon perforated zones. Actually, the origin of this problem is the short depth of investigation of the TDT tool (about one foot). So, any near-wellbore-effects may greatly influence the TDT log measurements. This problem was not solved by the new RST tool (C/O mode) because it provides a very short depth of investigation (6–8 inches), even shorter than that of the TDT. For these reasons, there was a necessary need to find another cased-hole log, which can overcome the limitations and restrictions of the Pulsed Neutrons technique. From field experience, it was found that both the used techniques (TDT, CHFR) have their own advantages and limitations and that each technique of them can be suitable for certain wells with certain conditions. The main objectives of the present work are to evaluate the new technique (CHFR) through actual field examples of sandstone reservoirs and to compare applications of CHFR and TDT techniques including technical and economical comparisons.