The primary challenge with weak production zones is severe to total circulation loss resulting from natural fractures or low fracture pressures. A highly thixotropic engineered cement system has been used successfully in the field in several severe-to-total-loss treatment procedures involving weak nonproducing zones. This paper presents an investigation of this cement system's response to acid treatment and the possibilities of extending its use as a temporary plug to cure losses in weak production zones while helping minimize formation damage.

Tests were performed to understand the response of the highly thixotropic engineered cement system to acid treatment. These tests included acid immersion of cured cement cubes and exposure of set cement to a continuous supply of acid by exerting a differential pressure in an acid core flow apparatus. A reacted cement core forms a conductive path, resulting in a subsequent decrease in differential pressure or an increase in flow rate. Dependency on temperature and differential pressure were studied.

Experimental observations indicated that there was a formation of conductive path caused by a cement reaction when subjected to acid treatment. However, complete dissolution is a prolonged process and is unlikely to occur within the typical acid treatment times used in the field. If sufficient differential pressure is provided to allow the acid to diffuse through the entire volume of cement, a regain in permeability can be easily achieved, even with short exposure times. It has been observed that the time required to form a wormhole decreases as differential pressure increases. When acid was replaced with water, no wormhole formation or increases in the permeability of set cement occurred. This confirms that the wormhole formation is a result of the chemical reaction of cement with acid.

The study presented in this paper confirms that the engineered cement system is amenable for acid treatment. This provides opportunities to use this cement system to treat severe and total losses, as well as provide wellbore strengthening in production zones. Such an operation helps ensure that the production zone is well prepared for a successful primary cement treatment without compromising production.

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