Highly Successful Acid Stimulation of a Vastly Heterogeneous Carbonate Formation Using Combination of Coiled Tubing and Non-Damaging In-Situ Foamed Diverter Gel
Authors: Sana-ul-Hussnain Muhammad , Malik Muhammad Ijaz , Anjum Zulfiqar Ali , Inam Sahir , Arif Muhammad , Shafee Imran , Haider Mateen , Elmoneim Hossam A. , Yousuf Arif , Khokhar Saad Y. , Anwar Rameez , Saeed Muhammad Salman 
Published: October 2023

A well "X" was drilled in a carbonate formation from no loss to total loss situation. Caliper log showed a huge thief/vuggy zone in the middle part of the formation. Based on the log interpretation, formation was found to be very promising. Formation was tested and found thick crude oil initially offloading in surges and eventually liquid flow almost ceased. In order to conclude the true potential of the formation, acid stimulation was become inevitable. But acid stimulation of this vastly heterogeneous carbonate reservoir was a challenging and critical activity therefore it require proper designing of treating recipes, temporary isolation of thief/vuggy zone, selection of diversion technique and in the end implementation for getting successful results.

Treatment of this challenging zone was designed in a step-by-step manner. Firstly, laboratory tests were performed on formation cutting and fluid samples for selecting treatment fluids. Laboratory results on fluid samples showed high compatibility with Xylene and mutual solvent mixture while cutting samples depicted excellent solubility results with HCl acid. Secondly, diversion technique was evaluated and thereafter non-damaging foamed diverter gel was selected for the thief/vuggy zone as chemical diversion. In addition to this, Coiled tubing as mechanical diversion was also selected for proper treatment of entire zone. Thirdly, the treatment was planned in batches/stages instead of continuously pumping one fluid at a time.

The whole treatment was successfully implemented as per the planned procedural steps while keeping all safety measures. After well cleanup, the well has started producing naturally at 950-1000 BPD of Oil with 0% water cut. A PLT survey was performed after the treatment, which has confirmed the success of this treatment. This excellent outcome of the treatment was mainly due to proper selection of the treating fluids and diversion techniques. Coiled Tubing was proved to be highly effective for placement and distribution of acid especially against the upper part of formation. While foamed diverter gel ensured in-situ diversion of the treatment along the complete pay zone especially against thief/vuggy zone. The use of foamed gel in stages helped to block the treated zone and force the next regular-acid stages to the untreated zones. Foamed gel/pill was not only temporarily isolated the thief zone but it also helped in flow back of treatment fluids.

This paper illustrates the philosophy for designing the acid stimulation and the requirement of both mechanical and chemical diversion techniques for proper acidizing of such a challenging well. This type of full-bodied technique was first of its kind for treatment of highly heterogeneous carbonate formations. The results of this technique were exceptional and similar nature of wells in future can be treated using this diverse approach.