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'Aberdeen' of India
India depends on several other nations to fulfill its petroleum energy needs. India imports more than 60 per cent of its crude oil requirement due to a huge demand-supply gap. India is the 9th largest crude oil importer and the 6th largest consumer of crude oil in the world. A large portion of oil is imported from the Middle East and African nations. India’s eastern coast is well known for reservoirs with extreme High Pressure/ High Temperature (HPHT) tight-gas formations, often referred to as 'Aberdeen' of India. Testing HPHT reservoirs and hydraulically fracturing with the same Drill-Stem Testing (DST) string is a big technological challenge. This article describes how technological development and a new approach have helped to test the extreme HPHT reservoirs in India with cost effective solutions so that the commercial viability of reservoirs can be determined. Exploring and testing such challenging zones and developing fields on the eastern coast of India will reduce the required import of oil and gas to some extent. This will also help the gross domestic product (GDP) growth. At present the oil and gas sector contributes over 15 per cent to the GDP of India.

Most operators face multiple challenges while testing such high pressure/ high temperature zones. When a major operator decided to evaluate reservoir potential in the East Coast Krishna Godavari (KG) Basin, it was recommended that all fracing or fracking (frac) and testing operations be combined into one trip, which will not only allow significant rig cost and time savings but will also provide the much needed flexibility for this critical operation. Many challenges had to be considered and mitigated for a single-trip operation, viz. reliable functioning of DST tools in extreme conditions for an extended duration, high pump rates through the DST string for effective proppant placement during hydraulic fracturing, limitations on downhole electronics from high temperature, annulus pressure limitations for annulus-pressure-responsive DST tool operations, and possible DST tool component damage during frac operations. Thus, DST tools with modifications such as a multi-cycle circulating valve with a debris tolerant mechanism, combinations of high temperature seals and a clear annulus fluid system to enable extended testing and frac operations with a higher success rate were deployed.
During multiple DSTs in HPHT environments and similar formations in India and globally, a major challenge has been observed in terms of debris settling in the ball and circulating sections of the multi-cycle circulating valve. This was addressed with tool modifications and resulted in the development of debris tolerant mechanisms. These mechanisms not only increased reliability of the tool performance in extreme conditions but also enhanced the frac operation through the DST string with high pump rate without affecting the operating capability of the equipment.

After reviewing the methods to combine all required operations into one trip, the operator decided to deploy a full suite of multi-cycle DST tools rated to 450 degrees Fahrenheit (°F). The proposed string would perform pre-frac testing, mini frac, main frac, and post-frac testing operations in a single run.

For the first run, tubing was run in the hole (commonly termed as the flex run) with a single shot tubing testing valve to flex the tubing using mechanical and electronic gauges. Generally, production tubing gets corroded and rusted when it is stored for a long period of time on rigs and at base. Flexing of the tubing helped to remove scale inside and outside of tubing, ensured restriction free inner bore, and also confirmed pressure test integrity of the tubing. The flex run also helped to determine static bottom-hole pressure and temperature. Static bottom-hole pressure data from this run provided the effective brine weight at DST tool depths under high temperature conditions. This accurateunderstanding of brine weight at packer/perforation depth and temperature helped to design the DST tools operating pressures precisely. After the flex run overbalance wireline perforation was carried out, the permanent sealbore packer was set on drillpipe.

For the main DST-frac run, multi-cycle DST tools along-with HPHT mechanical & electronic gauges were deployed. The DST string was pressure tested at various stages while running in the hole to ensure integrity of all tubular connections. After reaching packer depth, space out was performed. Flowhead along with the blow out preventer (BOP) safety valve was connected to the string and stung into the permanent packer sealbore. The BOP safety valve has a RAM lock section which facilitates closure of a 5” BOP RAM so that the annulus can be isolated and pressure can be applied for operation of the downhole valves. The final string schematic shown in Figure 1 shows at this stage of the operation, cushion fluid was spotted through the multi-cycle circulating valve and then the multi-cycle circulating valve was cycled to well test position and the well was opened for pre-frac testing. The multi-cycle circulating valve deployed was capable of unlimited cycles with annulus operating pressure and it would go through different positions as demonstrated below in Figure 2.
After pre-frac operation, a Diagnostic Fracture Injection Test (DFIT) was performed to understand hydraulic fracturing parameters such as In-situ stress, fracture closure pressure, and pump rates at various injection pressures. The operations performed as planned until the frac operation commenced, and injectivity could not be established due to gas migration and gel formation in the tubing. At this point, the multi-cycle circulating valve allowed reversal of the tubing contents by cycling it to the circulating position. After reversing out, frac fluid was spotted, the multi-cycle circulating valve was cycled to the well test position and the main frac operation was successfully performed. After main frac, a post-frac reservoir study was done. At the end of the post frac study, the well kill operation was carried out using the multi-cycle circulating valve.

It’s a common phenomenon to encounter fluid losses after well kill operation on the wells where a frac was recently performed. The deployment of the multi-cycle circulating valve would have allowed the isolation of the formation after well kill operations by cycling the tool to circulating position. This would have resulted in avoiding losses of the costly brine – clear fluid system into the formation.

SUMMARY
This job demonstrated that the technological advances and the lessons learned from a new approach allowed successful frac operations in a single run under extreme HPHT conditions. The DST tools maintained integrity, fraced through the string, and performed successfully to set a new record for the longest DST run under these conditions for the service company, and possibly, the industry. Accurate and reliable downhole pressure and temperature data was recovered from mechanical and electronic gauges. Significant rig time and an overall cost savings was achieved. This operation will assist in planning future frac operations in the high temperature – high pressure environment.