Increasing Cement-Sheath Integrity To Reduce Gas Migration in the Marcellus Shale Play
Authors: Carpenter Chris 
Published: March 2016

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 168650, “Cement-Sheath Durability: Increasing Cement-Sheath Integrity To Reduce Gas Migration in the Marcellus Shale Play,” by Jessica McDaniel, SPE, and Larry Watters, SPE, CSI Technologies, and Arash Shadravan, Texas A&M University, prepared for the 2014 SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, USA, 4–6 February. The paper has not been peer reviewed.
This study examines the effects of drilling, completion, and production operations and their associated cyclic stresses on a cement sheath. The operations performed after cement placement can damage cement-sheath integrity and the bond with the casing or formation, resulting in loss of zonal isolation and in sustained casing pressure, often requiring remediation and reducing productivity.
Introduction
The study reported here is a derivative of a long-term investigation aimed at improving zonal isolation for horizontal wells drilled in the Marcellus shale. The impetus for this long-term study is to optimize drilling and completion practices to reduce cost, improve zonal isolation, and improve well-success rate.
One of the initial actions of the study was assessment of well performance in the Marcellus play. Interestingly, the well-success rate was much lower in the intermediate string than in either the production casing or the surface casing. This observation was unexpected. The intermediate casings of Marcellus wells were set in straight hole at relatively shallow depth. Mixing and placement procedures were implemented similarly on all of these intermediate-casing cement jobs. Sufficient spacer volume, large excess cement volume, and cementing to surface ensured optimal drilling-fluid displacement. With no weak zones creating the risk of lost circulation and with normal drilling fluid and cement densities, adequate displacement rates, and routine use of gas-flow-control cement systems, the loss of zonal isolation in the intermediate/ surface annulus did not point to a short-term zonal-isolation issue. Rather, the potential flow path more likely resulted after the cement had set.
The loss of zonal isolation was noted after the drilling and completion processes, indicating that mechanical cement-seal damage might be leading to flowpath creation. Cyclic stresses applied to the intermediate-casing system, in the form of cyclic impacts from drillpipe whip during drilling or pressure stresses induced during fracture treatments pumped down the production casing, are considered to be potential stress sources. One significant result noted is the success rate of System 1 vs. System 2 (Fig. 1). This led to a thorough study of the mechanical properties and seal effectiveness of two intermediate cement blends, to test the mechanical-damage hypothesis. Durability and seal endurance of the cement compositions were compared and correlated to mechanical properties.
Results of the investigation support the hypothesis of gas-flow-path creation through or around the hardened cement. Comparison of mechanical properties, laboratory-seal durability, and field performance of two cements substantiate that cements developing higher tensile strength and other mechanical properties lower the incidence of sustained casing pressure. Initial attempts to correlate seal performance empirically as a function of applied stress were unsuccessful, but indications are that sufficient laboratory and field data will yield a correlation as a deliverable of the overall project.