The Raageshwari Deep Gas Field in the Western India, operated by Cairn, Oil and Gas vertical of Vedanta Limited, is a tight gas laminated reservoir with an average permeability of 0.1mD and a gross pay interval of ~700 metres. It is characterized by numerous packets of good porosity with high gas saturation. When the successful development of a reservoir is based on the ability to accurately model and install hydraulic fractures, verification of the fracturing models is critical. Verifying the fracture height is one way to reduce the uncertainty in the model results.

This paper describes how the Pulsed Neutron Logging (PNL) was used to determine the fracture height in near vertical well by detection of boron, a primary constituent of borate based crosslinked fluid systems. The discussion will also include the benefits of quantifying the actual fracture height such as determination of the required number of stages and calibrating the fracturing simulator.

Methods, Procedures, Process

The PNT log was designed as an alternative to temperature and radioactive tracer logs and has been successfully used for the identification of channels in the cement sheath surrounding the casing, fluid movements, gas migrations etc. in both production and injection wells. PNL is used to track the movement of a saline solution of borax and water by measuring changes in the capture cross section of an interval caused by the injected borax. The above technique involves running a base PNL pass, injecting a boron solution into the interval, and then making the post injection PNL passes. The boron solution generates a significant change in logging tool response, enabling the tool to identify solution movement inside and outside the casing.

The completion strategy in Raageshwari incorporates hydraulic fracturing through near vertical 3-1/2" monobore wells placed in 6" hole. Therefore, 8-10" depth of investigation of the PNL is sufficient to measure the frac height in the near wellbore area by measuring the boron present in the frac fluid in the fracture and the area adjacent to the fracture. Temperature logging was also performed in tandem with the PNL as an additional means of estimating fracture height.

Results, Observations, Conclusions

The pre-and post frac PNL survey responses were sufficiently different to enable estimation of the fracture height and the results were comparable to the heights obtained from the temperature logs. Knowing the fracture height provided an additional constraint which improved the fracture modeling. This technique is a cost effective way of determining fracture height without the need of pumping any additional hazardous (radioactive) or expensive materials. Its main advantage over temperature logging is its lower sensitivity to delays in the post fracture logging process.

Novel/Additive Information

The use of PNL for the fracture height determination from simple detection of boron present in the borate based cross linked fluid.

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