Fracturing Volcanic Rock in India: Continuous Improvements Over 11 Years
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This paper summarizes key engineering discoveries and technical findings observed during the execution of 200 hydraulic-fracturing diagnostic injection tests in the Raageshwari Deep Gas (RDG) Field in the southern Barmer Basin of India (Fig. 1 above). These tests were conducted in one of the few commercially viable thick and laminated volcanic gas reservoirs in the world. These diagnostic tests were spread over five separate campaigns over 11 years.
Because of the low permeability of this gas reservoir, hydraulic fracturing was necessary for sustained economic productivity. Because this massive laminated reservoir contained between 15 and 40 vertically separated pay sections, a key design consideration was to connect as much pay as possible with the least number of fracturing stages.
Although a conventional plug-and-perforation fracturing technique gives full assurance of optimal fractures for every bit of pay, the completion cost would undermine the project’s economics. Therefore, a limited-entry technique was selected. The uncertainties and risks were evaluated to maximize the probability of success.
More than 60 diagnostic fracture injection tests (DFITs), approximately 90 step-rate tests (SRTs), and approximately 50 minifracture tests have been conducted. In addition to conventional fracture diagnostics tests, other techniques were applied successfully. One such example was the use of multiple SRTs within the same fracture stage to evaluate limited-entry efficiency. As a result of the test data, the number of clusters per fracture stage was increased from three to six, achieving an overall increase in net-pay coverage of approximately 65%.
Hydraulic-fracturing operations and well flowback have several challenges in Rajasthan. Because this is an arid region, a continuous supply of water is problematic at best. In addition, the oilfield infrastructure is much smaller than typically seen in North America, with few suppliers and a dependence on small suppliers for periphery services such as water hauling. Because of these issues, the first campaigns suffered from significant operation delays and cost overruns. Key issues from previous campaigns were evaluated, and various plans were put in place to ensure smoother future operations.
With operational changes, an RDG Field 15-well program set new operational planning and execution benchmarks. The number of fracture treatments per month increased by more than 400%, while the cost per fracture treatment was cut in half. A summary of main operational challenges and their respective solutions is presented next.
Rig Up and Simultaneous Operations (SIMOPS). RDG Field development well pads have between 8 and 12 wells per pad. For the RDG Field 15-well campaign, the layout for rig up for fracturing and associated services was planned carefully to optimize the available area and to ensure maximum SIMOPS with minimum downtime.
Dual-Well Rig Up. In the previous campaigns, only one wellbore was worked on at a time, resulting in significant time lost from delays in bridge-plug running or the need for a wellbore cleanout because of excessive sand fill or screenout. However, in the last campaign, two wells were simultaneously lined up with a high-pressure manifold. Having an available backup well for fracture activities dramatically reduced nonproductive time and greatly improved the overall operational efficiency of the campaign.
Continuous Water Supply. In previous campaigns, small water tankers (35‑bbl capacity) were used to ferry water to the well pads. For the 15-well campaign, pipelines were used to carry water from a borehole in the production terminal to the various well pads. This eliminated all the downtime associated with water supply.
Fracturing Fluid. Using the borehole water introduced an additional problem. The borehole water was saline, and boron content was above the permissible level for crosslinking of the base gel. To use this water, a specially tailored fracturing fluid had to be developed. More than 60 laboratory tests were conducted before an acceptable fluid formulation was found.
Perforating. India’s regulations restrict perforating to daylight hours, which, combined with the greater-than-3000-m depth of the wells, resulted in additional time and operation costs. The current designs required as many as six runs of conventional perforating guns. This meant that executing more than one fracture treatment in 2 days was virtually impossible. Selective firing switches that allowed up to four gun firings in a single run were introduced as a solution.
Bridge-Plug Milling. The various fracturing stages were isolated from one another using composite 10,000-psi bridge plugs. These bridge plugs were milled using coiled tubing, with an expected milling time of less than 1 hour.
Near-Zero Gas and Condensate Flaring. Cairn India’s goal is to eliminate gas and condensate flaring when possible. Because the RDG Field is remote, previous well campaigns required some flaring for well cleanup. This flaring was deemed unacceptable, and all of the gas and condensate from the respective outlets in the separator was routed to the main production header with a nonreturn-valve setup. The separator pressure was used to push the gas and condensate into the header. It is estimated that approximately 3 MMscf/D of gas and 300 B/D of condensate were saved using this approach.
While a limited-entry technique was considered as an option to increase the net-pay interval covered by each fracture treatment, ensuring its effectiveness was also important. Various diagnostic tests were designed toward this end.
DFITs. DFITs were used to calibrate the minimum horizontal stress in the pay zones. After perforating the first cluster of a zone, a small injection test above fracturing pressure was performed. The pressure decline was monitored for an extended period. After the DFIT, the remaining clusters were perforated. The DFIT was performed on single clusters to eliminate interference between clusters and the corresponding uncertainty in analyzing the pressure decline.
Step-Down Tests (SDTs). Once all the clusters were perforated, an SRT followed by an SDT was conducted to evaluate the perforation pressure drop and efficiency. The primary objective of these tests was to determine whether the actual perforation friction was sufficient to ensure diversion into all of the clusters.
SDT analysis from Well P shows that approximately 2,200 psi of total entry friction and approximately 1,430 psi of perforation friction pressure were observed at the maximum pumping rate of 30 bbl/min. This is more than double the original estimate of perforation friction (600 psi), which indicates reduced perforation efficiency. Thus, fewer perforations are taking fluid and there is room to increase the number of perforations without adversely affecting diversion.
Multiple SDTs. Conducting multiple sequential SDTs was another method used to determine perforation efficiency. This technique was used in the second stage of Well J. First, three of the four clusters were perforated with eight holes each and an SRT was conducted to determine the number of open holes. This was followed by perforation of the fourth cluster and a second SRT. The entry friction at 30 bbl/min dropped approximately 900 psi. Because both SRTs were run at similar rates with linear gel, the only difference would be the additional perforation area. Analysis of this combined test indicated that 50–60% of the perforations were taking fluid. A temperature survey after the final SRT confirmed that all four clusters were taking fluid.
Evaluations such as these in more than 35 tests consistently showed higher entry friction. This provided the basis for increasing the maximum number of perforations from 24 to 30. Eventually, as many as six clusters per stage were placed successfully.
- Successful hydraulic fracturing in the RDG Field has been proved with more than 100 stages.
- A limited-entry technique offers an economical way to maximize reservoir contact in the RDG Field.
- Temperature logs show all clusters are being fractured.
- Production logs show that at least 80% of the clusters are producing and that contribution increases with time.
- The successful application of the limited-entry technique is dependent on maintaining the pressure drop throughout the job. For this reason, perforation erosion must be taken into consideration.
- When properly designed, as many as six perforation clusters can be treated at one time.
- Conventional fracturing models are adequate for designing and evaluating treatments in the RDG Field.
- A focus on operational efficiency can have a large effect on overall costs. At the RDG Field, the cost per fracturing treatment was cut in half between five-well and 15-well campaigns.
Fracturing Volcanic Rock in India: Continuous Improvements Over 11 Years
01 March 2018
06 March 2018
06 March 2018
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