Stability of an Openhole Completed in a Limestone Reservoir With and Without Acid Treatments
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Because a borehole in a limestone formation is more stable than expected, an openhole completion without a slotted/perforated liner has become popular recently. However, the following three items are not clear: (1) Why a borehole in a limestone formation is so stable, (2) why a borehole in a limestone formation can be completed without a liner regardless to the formation strength, and (3) the question of stability after acid treatments. To answer these questions, two types of laboratory experiments are conducted. One of them is a series of borehole stability experiments using 1.5- and 2.36- to 2.39-in.-diameter borehole in a 10.5×10.5×17.5-in. limestone blocks with polyaxial confining pressures simulating a horizontal well with three different principal in-situ stresses. Two types of limestones are used with and without borehole acid treatments and two borehole sizes are used to check the size effect. Another type of experiment is the acid squeezing experiment, in which 15% HCl acid solution is squeezed from one end of a cylindrical core and the change of porosity, permeability, and hardness are measured throughout the cores. The results showed the following new discoveries: 1. The limestones hav e two distinct failure envelopes. The failure plastic strain is relatively small for normal shear failure, while it becomes as much as 10 times larger when a shear failure is induced after pore collapse. 2. One of the limestones used in these experiments has o nly 1,751 psi UCS, yet the borehole was unexpectedly stable. The reason was that the borehole failure is induced by a shear failure after pore collapse. It is well known that pore collapse is induced within a formation during compaction; however, a shear failure after pore collapse has never been observed when one boundary is open like an open hole. 3. The confining stress inducing borehole failure was not significantly different between hydrostatic and directional loadings. According to the Kirsch's solution, the directional load should significantly increase the stress concentration. It is well known that the nonlinearity of rock reduces the stress concentration induced by directional loading; however, the present experiments showed that the magnitude of the reduction of stress concentration was larger than expected. 4. Wormholes stabilize boreholes even though acidizing weakens formation. Therefore, enhancing wormholes is recommended when a borehole in a limestone formation is acidized. Normally, because limestones are relatively strong, open holes are likely stable; however, the strength must be checked if they need to be completed without a linear protection. To help a reader applying the laboratory results to field problems, a guideline to complete an open hole without a liner protection in limestone reservoirs is provided, with calculation results using a nonlinear finite-element model. Copyright © 2005 Society of Petroleum Engineers.
author list (cited authors)
Morita, N., Doi, T., & Kinoshita, T.