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Case History: Underground Oil Storage Cavern Project

by Vinay Sahay

The use of water-based down-the-hole (DTH) hammer drilling is a relatively new cost-efficient technique. This technology has been used for the first time in India to make water curtain system in unlined cavern, in khondalite rock, for the Vizag underground crude oil storage cavern project.

The water curtain in storage tunnels provides an artificial water recharge in the rock mass in order to enhance flow patterns, and to maintain a high hydraulic potential in the storage cavern vicinity. During storage operation, the water curtain acts as a back-up to guarantee the underground flow requirements for the hydraulic containment of the stored product. The water curtain system in an unlined oil/gas storage cavern is most critical, as it helps prevent leakage and migration of oil/gas. The straightness of holes is essential to keep the uniform hydrostatic system (water pressure) above the oil/gas storage caverns, and to keep the higher hydrostatic system above the oil/gas storage cavern. A key advantage of water-based DTH system is that it helps to remove the air from the pore spaces of the rocks, otherwise it would be difficult to remove and can create problems – like becoming a low-pressure zone above the storage cavern, creating passage for oil/gas migration. During the course of drilling a borehole, the direction of the drill may change. The causes vary, however, the effect of changing formation hardness, compressive strength, structural conditions (dip, joint, fracture, foliation, etc.) and drilling technique are some of the major factors. Such deviation does not does not fulfill the purpose for which drilling may have planned.

Based on above reasons, it was proposed to use water-driven hammer (rotary percussion) drilling at the Vizag cavern project.

Geologic Characteristics
The Vizag underground storage project is meant for the storage of crude oil. Here, the lithology consists of khondalite suite rock, comprised of garnetiferous quartzo-feldspathic gneisses. This rock has higher hardness and uniaxial compressive strength (UCS). It is of higher abrasive grade due to the presence of quartz, garnet and coarse grain size. The purpose of water-driven DTH hammer drilling is to make 4-inch-diameter holes up to 330 feet in depth to be pressurized by the water – after completion of the cavern – to create a curtain of relatively higher water pressure to prevent migration and leakage of underground stored crude oil. The deviation of borehole path should not be more than 5 percent of the drill length. The borehole deviation survey was carried out with Reflex Ez trac, a multi-shot instrument.

Drilling Procedures
The Wassara W80 has been used to drill boreholes. The summarized results from drilling with water-powered DTH hammer are given in Table 1. As rock (khondalite) was hard with higher UCS, spiral stabilizers were used to keep the drill bit in a specified direction, and to maintain the straightness of borehole path. The convex-faced semi-ballistic button bits were chosen to be used, taking into account the properties of khondalite, well as moderate wear and tear of bits. Drilling results indicates that these bits are further helpful to get a straighter borehole path by making the hole faster, and minimizing the enlargement of borehole diameter, thereby decreasing the free space for the drilling tools to deviate. The blow count of the water-based DTH system is approximately 3,900 per minute, and thus, bits are less influenced by the jointed, small, cracked, lineated rocks, thereby minimizing deviation of the borehole path. With a combination of optimum frequency, the spiral stabilizer, convex-faced bits with semi-ballistic buttons, lithological-guided rotation speed and weight on bit, up to a 97 percent yield in straighter boreholes has been achieved.

Permeability Test
Once the hole is drilled to the full depth, then permeability tests will have to be carried out to check the permeability of the rock mass in the borehole with the help of a 3-inch- diameter pneumatic packer, pressure gauge and water meter to record the water consumption.

With water-powered rotary per-cussion drilling system, quite an efficient and economical result has been obtained. The results indicate that the water-powered down-the-hole (rotary percussion) drilling is a cost-efficient tool for production of non-coring drill holes with a straighter borehole path. 
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