Fluid Detection in Carbonate Reservoirs Utilizing Gas Analysis-A Case Study

Ali Abu Ghneej, Jalal Dashti, Badruzzaman Khan, Heyam Ammar, Abdulrazaq AL-Nabhan, Sunil K. Singh, Talal Al-Adwani, Nacif Marai

Explorationists have been continuously developing new technologies and techniques to achieve the goal of discovering Hydrocarbon. Gas detection while drilling and analysis of the data is very vital in evaluating the hydrocarbon potential of the formations. It is particularly significant in exploration of high pressure, tight, fractured carbonate reservoirs in Kuwait, which are drilled with Oil Base Mud (OBM) having high barite content causing severe formation damage.

This paper aims to present the procedure of interpreting gas data collected while drilling and their utility in formation evaluation, identification of productive intervals, fluid characterization and identification of gas/oil and oil/water contacts. Total gas and C1-C5 components readings, synchronized with depth and corresponding lithology, are acquired from the gas detector. The data is verified for any contamination, which may be due to mud containing oil, mud additive or pipe dope etc. The Gas Quality Ratio (GQR) is calculated to determine any contamination in the gas data. If the GQR is in the range of 0.8-1.2, the gas data is considered suitable to carry out gas analysis. As the reservoirs are drilled with OBM, the gas chromatograph gives abnormally high C5 against some sections which is not considered for the analysis. The good quality data is used to calculate various ratios in sections showing significant gas readings. Recently in few exploratory wells this gas analysis was carried out for Najmah and Marrat reservoirs. It has been observed that the more useful ratios in these carbonate reservoirs of Kuwait are C1/C1+C2+C3+C4+C5, Pixler ratios (C1/C2, C1/C3 and C1/C4), Wetness (100* C2+C3+C4+C5/C1+C2+C3+C4+C5), Balance (C1+C2/ C3+C4+C5) and Character (C4+C5/ C3). C1/C1+C2+C3+C4+C5, Pixler ratios and Wetness, Balance and Character indicate dry gas in the Upper section of Najmah followed by gas-condensate. These ratios indicate liquid hydrocarbon in Marrat. The results of the gas analyses were integrated with log and core data and were found to be very useful in identification of production test intervals. The gas analyses results have been ratified by production test results. Proper gas analyses on uncontaminated gas data can help to identify test intervals even in wells where log data could not be acquired due to well condition. It is recommended to record good quality gas data and carry out gas analyses in all exploratory wells.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012