Scrutinization and Evaluation of Heavy Sludge Formation and In-Situ Tar Mat Problems Based on Robust Integrated PVT and Open-Hole Logging Approach- A Case Study of an Oil Field in Pakistan
Authors: Siyal Amaar , Solangi Aftab Ahmed , Virk Muneeb Ali , Hameed Ali , Abid Hassan , Hassan Syed Saadat , Ameen Nadir 
Published: January 2024

Heavy oil is commonly produced in the form of water-in-oil emulsions. It has long been debated whether the emulsions are formed in the reservoir or inside the wellbore, and if so, what effect do they have on the recovery process. Meanwhile, sludge formation can significantly impair a well's productivity if deposited in the wellbore or at surface flow lines. In a field where sludge formation was not expected, the oil producing well showed a sudden deterioration in well productivity. Extensive lab 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 AB oil field in Pakistan based on a robust integrated approach. Secondly, to investigate whether the sludge formation is occurring within the reservoir or not.

For this purpose, an integrated robust workflow that was followed for the investigation of sludge/tar mat deposits in the wellbore and reservoir started with an investigation of PVT data of the oil field. PVT tests were conducted such as Saturates, Aromatics, Resins, and Asphaltenes (SARA) on samples acquired during the DST and after the sludge problem occurred. This was done to determine the content of asphaltenes and resins and their indirect affect on heavy sludge formation. This was done to identify the effect of asphaltenes and resins on the heavy sludge emulsion formation. In addition, the De-Boer approach was also used for the potential asphaltenes precipitation in the reservoir. Moreover, the Total Acid Number (TAN) and Water Analysis were also conducted for the possible identification of the effects of Naphthenates deposit and salts on sludge. Furthermore, the effects of different reservoir parameters i.e., Reservoir temperature, pressure, bubble point pressure, Gas-Oil Ratio (GOR), sulfur and wax content, oil API, and naphthenates-deposits were also highlighted. Finally, an open-hole logging interpretation along with PVT and wellbore modelling was done to highlight the possible compositional gradient, wax appearance temperature, and asphaltenes appearances within the reservoir.

The results showed that no compositional gradient or tar mat exist within the reservoir based on the micro-resistivity and mud-logging data as the separation between the deep later log and shallow resistivity was not broader. Meanwhile, no NMR log was available to confirm the presence of tar mat deposit within the formation and we can not rely solely on resistivity log. In addition, no thermal degradation and biodegradation of oil occurred in the reservoir as the temperature of the formation was below the threshold of 338 °F and higher than 122 °F, respectively. The sulfur and wax content along with depth were also far lesser from the threshold range of biodegradation which was confirmed through gas chromatography results. Moreover, the SARA analysis indicates a higher amount of resin content in comparison to asphaltenes which makes the oil more unstable and more prone to form stronger emulsion. Furthermore, the De-Boer method and PVT model indicate the reservoir pressure is above the asphaltenes precipitation window. While, the water and TAN analysis indicates that the ions concentration especially calcium and sodium were relatively higher while the TAN value was lower than 0.25 which was below the range of acidic crude which possibly indicates the formation of calcium Naphthenates that have caused the formation of strong sludge. Finally, PVT modelling and wellbore hydraulics indicated no compositional gradient existence within reservoir along with high salt drop out issue. No asphaltenes dropout was observed at the wellbore level.

The outcome of this research study will provide a way forward to identify and mitigate the strong emulsion root cause problem, which had caused significant decreases in the deliverability of the oil well. In addition, it also aims for providing a method for the screening of chemical de-emulsifiers, which will result in restoring and maintaining the well potential.