Characterization of the Early-Time Mechanical Behavior of Well Cements Employed in Surface Casing Operations

Abstract

Pressure events that occur after surface casing cementation, such as casing integrity testing, formation integrity testing, etc. all impose stress upon the recently set cement sheath. The magnitude of stress will depend on the pressure state, casing thickness, cement sheath thickness and mechanical parameters of the cement and formation. Should pressure testing take place during the early stages of cement curing, the tangential stress imposed by the pressure event can exceed the tensile strength of the cement, thereby inducing cement sheath failure. It is well documented that cement is much stronger in compression than in tension. In most wellbore pressure scenarios, cement fails in tension. The proportionality between the compressive strength and the tensile strength of set cement is generally assumed to be an 8:1 to 10:1 ratio. During typical pressure testing events, the cement will have a compressive strength ranging from the 500 psi required for the commencement of drilling operations to upwards of 2000 psi depending on the cement curing time. Accordingly, conventional wisdom would hold that the tensile strength of the cement would be in the range of 50 to 200 psi at the time of casing pressure testing. However, accurate prediction of the degree of pressure induced cement sheath stress requires more than a general correlation to derive cement tensile strength. This paper characterizes the early-state physical properties and mechanical behavior of accelerated API Class A, G, H and ASTM Type I cement designs during the twelve hours following placement. The confined/unconfined compressive strengths, ultrasonic compressive strengths, tensile strengths, Young's Modulus and Poisson's Ratio of four commonly used "tail" cements as a function of time are presented. Predictions of induced stress in typical casing/hole size combinations as functions of pressure are also included. The results from this testing provides guidance as to when pressure testing of the casing/formation can take place without inducing damage to the set cement sheath. Introduction Surface casing used in well construction serves to isolate unconsolidated formations and fresh water aquifers found at shallow depths. The surface casing prevents contamination of groundwater by drilling fluids used in the well and produced fluids such as oil, gas or brine. Additionally, the surface casing is the structural foundation of the well, as it is often the first casing string to which the blowout preventers are connected[1]. Surface casing cement performance requirements such as compressive strength at time of drill out or at a particular time interval, free fluid, etc. are stipulated by the responsible state or federal regulatory authority. Typical surface casing cement designs incorporate economical volume extended slurries mixed at 11.5 - 13.5 lbm/gal ("lead"cement) followed by a "tail" cement mixed at 14.8 - 16.5 lbm/gal (depending on cement type) that is placed in the lower section of the casing-wellbore annulus. Accelerators, such as calcium chloride, are often used to reduce the slurry thickening time and enhance early compressive strength development, thereby minimizing waiting on cement (WOC) time. Once the cement is in place and set, maintaining annular isolation will depend upon the mechanical behavior of the cement and formation and the stress conditions under which the cement sheath is placed. Post-cementing stress imposed upon the cement sheath is most often the result of a change in the pressure environment due to obligatory casing pressure testing, formation integrity testing or a change in wellbore fluid density. Recent efforts within the well cementing industry have focused attention on the long-term competency of the cement sheath under the stress conditions expected during the life of the well[2–7]. For the most part these investigations have centered on stress induced events found deeper in the wellbore or during the productive phase of the well history. The early state physical properties and mechanical parameters of cement employed in surface casing applications have not been reported in the literature.

Experimental Evaluation of Stability and Rheological Properties of Foam Cement for Oil Wells

“…It involves placing a cement slurry in the annulus between the well casing and geological formations to create a solid mass for supporting and sealing (Thiercelin et al, 1998;Singamshetty et al, 2004;Hernandez and Nguyen, 2010;Ibrahim and Almahdawi, 2016). The cement's ability to maintain annular isolation depends on the formation's mechanical behaviour, cement, and stress conditions (Mueller and Eid, 2006). The cement must provide enough strength to sustain the casing and resist the stresses it will encounter over its lifecycle (Smith, 1991).…”