Summary Loss of zonal isolation in a wellbore can be caused by mechanical failure of the cement or by development of a microannulus. However, behavior of the sealant is driven by specific boundary conditions such as rock properties. Large-scale laboratory testing of the cement sheath in an annular geometry and a confined situation was performed to simulate various downhole stress conditions and evaluate the behavior of several sealants. Failure modes of the cement sheath were determined as a function of cement mechanical properties, loading parameters, and boundary conditions. Results were used to validate an analytical model that predicts cement-sheath failure. Introduction Interzonal communication in a wellbore may lead to loss of reserves, contamination of zones, production of unwanted fluids, or safety and environmental issues. Remedial solutions exist to repair the problems, but for technical or economic reasons, the well may be shut in or abandoned. To maximize well life, the cement sheath must be chemically and mechanically durable. Sealants resistant to aggressive formation fluids should also be designed to withstand stresses exerted during production and well operations, such as casing-pressure tests, stimulation treatments, or temperature changes during production cycles. To achieve this design goal, a better understanding of the mechanical behavior of different sealants under downhole conditions is required.1,2 According to Thiercelin et al.,3 changes in downhole conditions can cause mechanical damage (e.g., mechanical failure or creation of microannuli) to the cemented annulus, which may lead to loss of zonal isolation. Thiercelin et al.'s paper3 concludes that the complete mechanical system formed by the steel casing, cemented annulus, and formation should be considered, rather than sealant strength alone. Increase of pressure and temperature in the wellbore first expands the inner steel casing, which instantly imposes this deformation on the surrounding cement sheath. This applies imposed displacements, rather than imposed stresses, to the cement inner diameter (ID). Over the lifetime of the well, the cement sheath must withstand multiple displacement cycles. Several authors4,5have proposed numerical models to simulate sealant mechanical behavior and predict initiation of failures according to known mechanical properties of the complete system (i.e., steel, cement, and rock). A large-scale laboratory test for sealants in an annular geometry has been developed. This test simulates changes in well conditions that cause contraction or expansion of the inner casing. It can also evaluate the confining role of the formation or outer casing. Such an experiment enables the evaluation of sealant mechanical responses under wellbore conditions. The tensile and compressive stresses generated in the annulus are similar to those the sealant must withstand in a real wellbore. Loading simulated in the full-scale annular sealing test is close to real field conditions. Several cement systems exhibiting different mechanical behaviors have been tested, and the experimental results have been compared with predictions of a numerical model.
In a wellbore, loss of zonal isolation can be caused by the mechanical failure of the cement or by the generation of a microannulus. However, the behavior of the sealant is driven by the specific boundary conditions like the rock properties. Large-scale laboratory testing of the cement sheath in an annular geometry and in a confined situation was performed to simulate various well conditions and to evaluate the behavior of several sealants under simulated downhole stress conditions. The failure modes of the cement sheath were determined as a function of the cement mechanical properties, loading parameters, and boundary conditions. The results were used to validate an analytical model that predicts cement sheath failure. Introduction Interzonal communication in a wellbore may lead to loss of reserves, contamination of zones, production of unwanted fluids, or safety and environmental issues. Remedial solutions exist to repair the problems, but for technical or economical reasons, the well may be shut in or abandoned. To improve the lifetime of the well, the cement sheath must be chemically and mechanically durable. Sealants resistant to aggressive formation fluids are designed when required. In the same way, sealants should be designed to withstand the stresses experienced during production and well operations - e.g., casing pressure tests, stimulation treatments, or temperature changes during production cycles-throughout the well life. To achieve this, a better understanding of the mechanical behavior of different sealants under downhole conditions is required to design fit-for-purpose materials.1,2 Several papers have been written on the subject. According to Thiercelin et al.,3 changes in downhole conditions can cause mechanical damage to the cemented annulus (mechanical failure or creation of microannuli) that may lead to a loss of zonal isolation. The key conclusion of that paper was that instead of considering the strength of the sealant as the main property, one should rather look at the complete mechanical system formed by the steel casing, the cemented annulus, and the formation. Indeed, increase of pressure and/or temperature in the wellbore firstly expands the inner steel casing, which instantly imposes this deformation on the neighboring cement sheath. As a consequence, imposed displacements rather than imposed stresses are applied to the cement inner diameter (ID). At a greater time scale (the lifetime of the well), the cement sheath must withstand multiple displacement cycles. Several authors have proposed numerical models4,5 to simulate the sealant mechanical behavior and predict initiation of failures according to known mechanical properties of the complete system (steel, cement, and rock). A large-scale laboratory test for sealants in an annular geometry has been developed. Changes in the well conditions resulting in either the contraction or the expansion of the inner casing can be simulated. Furthermore, the confining role of the formation or outer casing can be evaluated. Such an experiment allows the evaluation of the sealant mechanical response under wellbore conditions. Indeed, the nature of stresses generated in the annulus (tensile and/or compressive) is similar to those the sealant must withstand in a real wellbore. The loading scenario simulated in the full-scale annular sealing test is close to reality. Several cement systems exhibiting different mechanical behaviors have been tested, and the experimental results have been compared with the predictions of a numerical model. Laboratory experimentation The experiments are designed to compare different cement formulations at room conditions in a large-scale annular geometry and determine the effect of cement mechanical properties and boundary conditions (rock stiffness) on cement cracking and permeability to air. Imposed deformations can be applied on the cement ID to simulate changes in wellbore conditions caused by variations of temperature and/or pressure. Equipment The equipment developed for the study is shown in Figs. 1 and 2. There are two main components.
Conceptual models are used to solve specific problems in selected sectors of reservoirs; study production mechanisms; understand behavior of a particular process in a reservoir system, and assess impacts of changing input parameters during reservoir modeling. They are tools of choice for assessing risks, evaluating "worst-case" scenarios, validating analyst's intuition, and to support informed decision making. Our objective is to demonstrate via two case studies how conceptual numerical models were used to shorten the time required to make reservoir management decisions. The first case study involves making a decision, either to develop or sell an oil property. Target formation is sandstone saturated with heavy oil (12°API gravity) which is overlain by a gas cap. Conceptual numerical simulation models provided answers to two questions:What is the impact of gas production from the gas cap on the underlying heavy oil zone?Can gas production from up-structure wells meet field deliverability requirements? Second case study uses conceptual models to optimize well placement and support infill drilling. Infill well placement posed a challenge because thickness of target formation is not well known, and oil zone is bounded on top by a massive impermeable shale boundary, and by oil-water contact (OWC) located about 20–40 feet below. Conceptual models answered the following questions:What type of well to drill--vertical or horizontal?What is the impact of horizontal well's vertical placement (offset distance from OWC) on oil recovery and water breakthrough times?What is the optimum horizontal well lateral length and its impact on oil recovery? This paper describes modeling methodology, major observations and conclusions. We discuss the benefits and lessons learned from the case studies and demonstrate that successful application of conceptual models requires identifying key well/reservoir performance drivers and assessing their impacts on the reservoir management decisions. Introduction Conceptual models are designed to solve specific problems in selected sectors of the reservoir. These models are built to study production mechanisms, understand the behavior of a particular process in a reservoir system, and evaluate the impact of changing input parameters during reservoir modeling. Conceptual models can be used as standalone tools for reservoir studies or can be incorporated into the work flow for full field modeling. They are the tools of choice for assessing risks, evaluating the "worst-case" scenarios, validating the analyst's intuition, and to support informed decision making.
Over the past ten years, several papers have been published discussing the long-term mechanical durability of the cement sheath. The customary procedure is to use a model to predict potential failure scenarios and to subsequently design a sealant material that will not fail under the expected conditions. The predictive models are either analytical or finite-element models. The analytical models can only be applied to relatively simple situations that require a simplified set of input data. In these cases, the results are consistent with those of finite-element models. More complex situations can be simulated with finite-element models, but the input data requirements are far more severe. Typically in the modeling papers, little information is included on how the input data is obtained. Because of this, several papers have been published that proposed ways to obtain the input data, in particular the mechanical parameters of the set cement. However, because these papers have typically addressed only one or two parameters, the proposed methods are inconsistent. This paper critically reviews the published information; highlights the strengths and weaknesses of the various approaches, and recommends a self-consistent set of measurement methods. The predictions from the given approach have been verified at the field level through evaluation of actual wells. Introduction The effect of pressure and temperature changes on the integrity of the cement sheath was demonstrated experimentally many years ago.1,2 More recently, this behavior has been modeled using both analytical3 and finite-element4 models. Athough finite-element models are capable of handling more complex situations, they are usually used with simplifying assumptions in which case they offer no advantages over analytical models. Indeed, Bosma et al.4 used the analytical model and results from reference 3 as a benchmark for their finite-element model and showed good agreement between the two models. Formation creep, which can potentially induce large strains in the cement sheath, was also discussed by Bosma et al., but this will not be addressed in this paper. Both types of models assume a linear elastic mechanical behavior. Therefore, the response of the cement sheath to strain is determined by the static Young's modulus and Poisson's ratio of the cement through Hooke's law. A correction of dynamic values of Young's modulus and Poisson's ratio is required to be used in Hooke's relationship. The failure point is given by either the tensile strength or the compressive strength of the cement (using the Mohr-Coulomb failure criterion for the latter), depending on the expected failure mechanism. In general, decreasing the Young's modulus or increasing the Poisson's ratio of the cement will decrease the stresses induced in the cement sheath and, for a given situation, will decrease the risk of failure. In the modeling papers discussed above, there has been little discussion of how to determine the appropriate parameters that describe the cement mechanical behavior. Thiercelin et al.3 determined the Young's modulus in flexion and flexural strength (Mr) from three-point bending tests. The authors noted that the loading rate is a key parameter in determining the ultimate strength of the material: the lower the loading rate the lower the flexural strength measured. They applied a safety factor of 50% to the flexural strength to obtain a tensile strength value more representative of downhole conditions. However, as three-point bend tests were performed, there was no way to determine the Poisson's ratio of the cement; so, the value was estimated at 0.2. The model described was a linear thermo-elastic model, so the use of a single value of Young's modulus and Poisson's ratio was appropriate. Bosma et al.4 used confined triaxial and unconfined uniaxial measurements to characterize the cement behavior but did not give details of the experimental methods used. Although the authors discussed nonlinear failure behavior, the prefailure behavior was described by a single value of Young's modulus and Poisson's ratio, suggesting a linear-elastic model was used. This lack of discussion on the test methods used to determine the cement behavior input data has been recognized by several authors, and there have been several papers published that discussed ways to measure mechanical parameters of set cement, but none have provided a complete package of measurements.
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