Robust Integrated Approach Evaluates Heavy Sludge Formation

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 219501, “Scrutinization and Evaluation of Heavy Sludge Formation and In-Situ Tar Mat Problems Based on Robust Integrated PVT and Openhole Logging Approach: A Case Study of an Oil Field in Pakistan,” by Amaar Siyal, Aftab Ahmed Solangi, SPE, and Muneeb Ali Virk, Mari Petroleum, et al. The paper has not been peer reviewed. _ In a field where sludge formation was not expected, an oil-producing well showed a sudden deterioration in well productivity. Extensive laboratory analysis indicated that sludge deposition was promoted by the presence of asphaltenes, resins, high amounts of calcium and sodium contents, and low-pH brine. The scope of this work was to investigate the root cause of strong oil/water emulsion and sludge issues of the AB oil field in Pakistan based on a robust integrated approach. Field Overview The AB field was discovered in December 2018 and was put into production in May 2019. The well is perforated in the Khewra sandstone reservoir, which is a four-way fault-bounded structure. The initial reservoir pressure of the field was 8,140 psia, with a reservoir temperature of 276.5°F. The reservoir has a strong aquifer drive mechanism with an API of 28°. At the beginning of the production, the water cut was low. Within a month, however, a dramatic increase in water cut was observed, resulting in severe issues of sludge and oil/water emulsion. Currently, the well is operating with 60% water cut at an oil rate of 120 B/D. The fluid type is characterized as black oil with a bubblepoint pressure of 1,160 psia. Methodology The pressure/volume/temperature (PVT) data of the whole AB field was analyzed in which saturates, aromatics, resins, and asphaltenes (SARA); total acid number (TAN); gas chromatography; and sulfur and wax content were evaluated. Various reservoir and fluid parameters of the field also were investigated. Then an auxiliary approach [i.e., the De-Boer method, colloidal-instability index (CII), and resin/asphaltene ratio (R/A)] were used to investigate the origin of sludge formation. To determine whether the sludge or oil/water emulsion was being formed within the reservoir, an openhole logging approach was taken to evaluate whether a compositional gradient exists within the reservoir. Furthermore, to evaluate the effects of salts on the formation of sludge, TAN and water analysis were performed. Finally, wellbore-hydraulics modeling was performed. Results and Discussion Several subsections of the complete paper detail the results of a variety of investigations, evaluations, and tests; a few of these subsections are presented in this synopsis. PVT Data Evaluation. PVT data were analyzed and evaluated. The four bottomhole samples were collected and initial quality check and validation was completed. The moisture content of these samples was measured by the Karl Fischer method. Two samples were completely of water. A sample with less moisture content and a volume of free water drained was chosen. Sample 2, with 8,410 psia at 276.5°F, a volume of free water drained of 180 cc, and a moisture content of 0.24, was chosen for conducting the PVT and other tests. Then, compositional analysis of flashed liquid using gas chromatography was completed. Results showed that the fluid type was black oil, having fewer heavier components. No biodegradation of oil was observed. In this case, the complete suite of n-alkanes was intact, and n-alkanes are greater than adjacent isoprenoids. Moreover, no significant hump in the unresolved complex mixture of branched and cyclic compounds was observed.