Unlocking the Production Potential in Mature Fields: A Systematic Approach to Acid Stimulation
Authors: Olufisayo Fagbowore , Dawari Charles , Lewis Williams , Femi Odusote 
Published: October 2018

Abstract
Acid stimulation has been used extensively to improve well productivity, as early as the late-19th century. The method was first applied to carbonate reservoirs. Over the years, it has seen application in sandstone reservoirs. However, it has been more of a hit-or-miss in terms of result. Success and failure of acid stimulation has often been attributed to candidate selection (Nnanna et al, 2009)1.
The NNPC/CNL Joint Venture is made up of a total of 26 producing fields in the Niger-Delta basin of Nigeria with over 380 completions currently on production. Focus on production optimization efforts has helped to get the best from the wells, some of which have been on production since 1965. Over the years, production has declined in these matured fields for several reasons; reservoir pressure decline, reduced reserves, increased water and gas production, and near-wellbore damage among others. In cases where well productivity has declined over time in reservoirs with minimal drop in pressure and significant remaining reserves, acid stimulation has been identified as a method to boost productivity. Efforts were therefore made to identify acid stimulation opportunities in a cost-effective manner.
With the paucity of cases when acid stimulation has been successfully used to improve oil production in these fields, it becomes pertinent to develop processes to select the best candidates that can yield incremental production with properly designed acid treatment. Also given the large number of completions available to be assessed, a set of technical selection criteria become important to identify potential underperforming wells with near-wellbore damage prior to carrying more detailed analyses like the classic Pressure Transient Analysis. This becomes more important in the current oil price regime with the limitations on spending.
With over 180 well-head jackets spread over a total of 2.2 million acres in the JV, key challenges had to be addressed for optimal execution of the stimulation campaign. The mode of acid placement, selection of equipment and order of job execution were decisions that had to be made to arrive at the most economic but operationally successful execution strategy for the campaign.
From a shortlist of 19 candidates, a 3-well pilot campaign was successfully executed using this approach with a total production gain of over 1900 BOPD. This paper discusses the detailed selection criteria used, the results of the pre- and post-stimulation characterization of the near wellbore damage using Pressure Transient Analyses, which supported the selection criteria and the holistic technical assessment carried out to determine the best treatment methodology for the candidates. The importance of a full team collaboration will also be highlighted. The results of the campaign, the lessons learned and the best practices based on the outcomes of the execution are documented alongside the operational challenges encountered and how these were managed.

Background
The NNPC/CNL Joint Venture is operated by Chevron Nigeria Limited (a Chevron affiliate company) with acreage covering both the onshore and offshore areas of the Niger-Delta (Figure 1). First oil discovery was in the Okan Field in December 1963. The JV commenced production in 1965. Currently, 26 oil and gas fields are producing from the Niger-Delta basin.
The targeted reservoirs in these Niger-Delta fields are located within the NW-SE oriented Pliocene to recent depocenter, in wave and tidal dominated delta. The Basal Akata marine shale serves as the source rock for the overlaying Agbada paralic sandstone reservoirs. The system is overlain by continental to shallow marine sandstones of the Benin Formation (Figure 2).