Formation compaction has significant physical and economic impacts in the drilling, production and asset management of hydrocarbon reservoirs. Outside of heavy duty well construction activities or controlled production that may not be economically viable, little seems to be done in countering subsidence in the well settings typically found in the Gulf of Mexico and elsewhere in the world. Recently, however, several wells in offshore Gulf of Mexico blocks were completed with radioactive marker tags placed within actively producing reservoirs. These tags were placed strategically onto the production casing at regular intervals.
A surveillance logging program has been conducted for these wells beginning with 1996 initial baseline surveys in some wells. Several acquisition methods have been undertaken and studied in order to derive an optimal measurement technique. In a few cases, surveillance included four or more independent snapshots in time. The purpose of this program has been to better understand subsidence within specific geological horizons. The effort has resulted in an improved understanding based on individual wellbore measurements and with the benefit of field based proprietary simulations.
Comparisons of these findings are shown, and the paper also reports on the development of best practices for logging and on methods for quality assessment and evaluation of measurements. Findings from these activities are summarized in terms of the utility and justification for conducting such an extensive program. Trends between the survey findings and other methods for evaluation of the effects of compaction and subsidence in producing reservoir are discussed. The value derived from these measurements justifies similar compaction acquisition approaches for subsequent monitor surveillance in wells in the Gulf of Mexico and globally.
Selected well examples highlight the results of log based monitoring of subsidence within reservoirs and the strain associated with in-reservoir subsidence over time. The results include time-lapse strain profiles and displays that also illustrate the quality of repeat measurements. From recent studies of the surveillance findings there have been enhancements made in the data acquisition sensors and expansion of measurment applications. These very recent changes are explained within the paper and may lead to compaction surveillance in a much broader spectrum of wells than are currently involved in this activity.