Due to the stacked nature of reservoirs in the Niger Delta, the predominant completion types are dual-string multizone and single-string multi-zone completions. These designs have been adopted to reduce the number of infill wells required for field development. However, they come with a disadvantage in regard to carrying out a successful intervention when water break through occurs.
Water breakthrough and high basic sediments and water (BS&W) are problems associated with fields having strong aquifer drive mechanisms. As a result, most exploration and production companies have learned to manage water production up to a tolerable limit, which is dependent on the water handling capacity of the installed facilities and also the economic cutoff limits for the wells in question.
The reason for this type of water management is the lack of confidence in the water shutoff remedial operations. From a survey carried out in the early 90s, it was estimated that only 35% success was achieved worldwide in water shutoff remediation. This low success rate is due to poor diagnosis, wrong selection of water shutoff solutions, and how complicated the well completion is with respect to the zone of interest to be treated.
Field X, 1, 2, which consists of a large gas cap and a 100-ft total vertical depth (TVD) oil column, was developed with the single-string multizone completion design. Due to the presence of a strong aquifer in this field, water production started early and some of the wells were shut-in due to lift problems associated with the water production. A sidetrack option was considered as a means of bringing these wells back on production, but was not used because of the absence of a gas gathering facility for the field. As a result of production decline and lack of infill opportunities, cement-water shutoff and re-perforation intervention in the wells was adopted.
The objective of the cement-water shutoff was to ensure that the perforations, which were flushed, were completely sealed off and isolated and subsequently re-perforated shallower.
After slurry placement and squeezing, it is important to ensure that a good cement job has been performed. Operationally, the top of cement (TOC) is tagged using slick-line in a vertical or deviated well. If the TOC is not at the theoretical depth, then a top-up job is carried out with additional slurry. On the other hand, if the TOC is at the theoretical depth, then a pressure test is performed to confirm that the perforations are squeezed off.
For intervals behind the sleeve as in the case of Field X, ascertaining the TOC is technically impossible because the perforations are behind the production tubing. For such single-string selective completions, only a pressure test can be performed to confirm that the perforations are squeezed off. This paper addresses the planning, operational and the learning from the through-tubing water shutoff campaign successfully carried out on wells with single-string multizone completions.
Cement-water shutoff intervention behind the sleeve in multizone completions is a solution that is not common due to its low probability of success. Shell Petroleum Development Company Nigeria and Schlumberger successfully carried out this operation in four wells drilled and completed in Field X.
The biggest issue associated with cement squeeze in a singlestring multizone completion is the difficulty associated with placement and confirming where the TOC would be after the intervention. This by implication makes it difficult to determine if a good cement job has been performed.