Cement Evaluation Case Studies; Application of Multiphysics Measurements to Address Different Challenges in Deepwater Gulf of Mexico Environment

Abstract

The Gulf of Mexico (GOM) is home to thousands of deep water wells in a variety of status such as production, temporary or permanently abandoned. Regardless of the status of the well, the condition and integrity of the cement sheath behind casing is critical for the safety, performance, economics and life span of the well. For temporary or permanent abandonment, the condition of the cement sheath is an integral part of the overall risk assessment and associated well controlled measures designed to contain and prevent migration of undesired fluids to surface. Challenges related to the GOM environment highly influence the successful placement of the cement behind the casing and its subsequent evaluation. With typical water depths in excess of 2,500ft., a wellbore trajectory can reach and exceed 30,000ft measured depth and can be highly inclined. Hydrocarbons bearing formations may be unconsolidated sands with high risk of sand production. Within the same wellbore, a succession of pressure-depleted zones, water zones, hydrocarbons zones, high pressure zones, faults, etc. may be encountered. The typical completion strategy is referred to as cased-hole frac-packing. This completion methodology provides substantial benefits in alleviating potential sand production, reservoir fluids production stabilization, and reduction of near wellbore skin by creating network of high-conductivity channels deeply into the formation. However, the creation of these deep fractured-channels poses additional risks for out-of-zone fracture growth along the wellbore in the absence of a competent cement sheath. Different types of cements and placement techniques are used in the GOM. These cements are specifically designed to address each challenge expected in any wellbore–highly depleted zones (thief zones), over-pressured zones, unconsolidated high porosity/permeability zones, highly inclined wellbores, etc. In addition to the complexity of the different cement recipe, the conditions inside the wellbore are of equal importance during the cement evaluation–borehole size, borehole fluid (oil vs. water), borehole fluid density, solids content, gasified fluids, etc. This paper focuses on two cement evaluation technologies. The first technology, based on an ultrasonic compressional wave, presents the distinctive advantage of reducing borehole fluids influences such as highly attenuating oil based or synthetic muds. The second technology uses innovative electro-magnetic acoustic transducers (EMAT). The EMAT acoustic signal is generated directly in the casing wall, making the measurement wellbore-fluid neutral. Multiple wave modes enables qualitative and quantitative interpretation in a wide range of slurry densities, including light-weight and foam cements. This technology provides the industry most direct solid behind-pipe evaluation through the attenuation of a shear wave which is a unique indication of solids. Selection of either technology, provides GOM operators with added confidence in evaluating cement sheath based on expected conditions, avoiding costly unusable log data.