Casing/cementing/zonal isolation

Design Procedure for Cementing Intercalated Salt Zones

This work demonstrates cement design that includes evaluating cement-sheath mechanical integrity in intercalated salts.

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Source: Getty Images.

Wells often require being drilled through and cemented across salt formations. In many parts of the world, salt sections consist of multiple salt types. This analysis shows that intercalated salts subject cement sheaths to a series of tensile and compressive loads whose magnitude depends on the size and relative position of different salts. The salt/salt-interface effects dominate the general tenet of increasing creep rate with increasing depth. This work demonstrates cement design that includes evaluating cement-sheath mechanical integrity in intercalated salts.

Introduction

Drilling and cementing challenges associated with salt formations are well-known. One of the more significant of these is the plastic deformation of salt attributed to the existence of deviatoric (shear) stress. This deformation is known as creep.

To determine the role of creep in the mechanical integrity of a cement sheath, it is necessary to analyze the thermostructural model of the well-construction process using the creep constitutive relationship. The outcome of the analysis is the stresses experienced by the cement sheath. It is possible to quantify the risk posed by salt creep and other operational loads to the cement-sheath integrity by comparing these stresses to the failure properties of the cement. To provide an accurate quantification of the risk, it is necessary to verify the creep constitutive relationship with data from experimental creep tests.

The type of failure of cement sheath depends on the type of loads exerted on it. For example, compression loads are likely to result in shear failure. Similarly, extensional loads are likely to result in tensile failure. In a typical well-construction process, the in-situ stresses attributed to overburden are greater than the hydrostatic pressure offered by wellbore ­fluids, such as drilling mud, cement, and completion fluid. Hence, the deviatoric stress on salt formations will lead to wellbore closure. The cement sheath thus will experience compression loads caused by creeping salts. However, the compression load is not uniform along the axial direction. This is because the creep rate of salt varies with depth. The variation is severe when multiple salts with different creep rates are intercalated. Fig. 1 shows an example analysis of openhole closing in the presence of intercalated salts. In such scenarios, the salt/salt-intercalation junctions exert extensional loads on the cement sheath. The closure is not to scale, and the displacements are magnified for a better view. Thus, in the presence of intercalated salt formations, the possibility of tensile failure of a cement sheath should be investigated in addition to shear failure. Evaluating for both shear and tensile failure requires analyzing a longitudinal wellbore model.

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Fig. 1—(Left) An axisymmetric view of a virgin open hole consisting of halite (red), tachyhydrite (yellow), presalt, and post-salt (green) formations. (Right) The deformed state of this wellbore after a finite exposure time to a predetermined mud weight.

Details

The design procedure is explained by use of a typical example well from offshore Brazil. Analysis is then used to compare the response of two cement systems. A validated creep model is used to strengthen the analysis predictions.

Creep-Model Details. There are different forms of creep models covering the three types of creep regimes—primary creep, secondary creep, and tertiary creep. This paper uses a two-mechanism creep law to describe the secondary-creep process because this is the dominant creep regime for downhole applications. Please see the complete paper for a presentation of the creep equation.

Design Procedure. There are two parts to building a design procedure. The first is building a geometric representation of the pertinent part of the wellbore model along with the correct creep characteristics of the formations selected. The second is building the analysis procedure that reflects the loads exerted and their sequence.

Building Geometric Model. The well being analyzed had small sections of tachyhydrite and anhydrite intercalated with long sections of halite. The entire salt section is approximately 2020 m thick.

Analyzing the entire salt zone is computationally prohibitive. So, a critical section of interest that forms the geometric model should be identified. The procedure to identify the geometric model is as follows:

  • Identify the tops of each formation.
  • Determine the average temperature and in-situ stresses for each formation.
  • Calculate the thermal averaged creep rate from the temperature-dependent creep properties and in-situ stresses.
  • Identify the contiguous layers exhibiting the fastest creep rate. This forms the salt section of interest.
  • To this salt section, add a small formation above and below to represent the overburden and bottom support, respectively. This forms the geometric model.

Building Analysis Procedure. The analysis procedure requires the sequence and duration of each stage during well construction. The final stress state from a stage forms the initial stress state for the following stage. The typical construction process involves the following stages in the order that they are defined: drilling, running casing, cementing, waiting on cement, pressure testing, and production. In each of these stages, the thermal and structural loads experienced by the formation, the cement sheath, and the casing will change. Because of the longitudinal nature of the wellbore model, there is an axial variation of stress and wellbore pressure.

Cement-Systems Details. Tested mechanical properties of two cement systems are compared. The first system, Cement System 1, is a salt-saturated slurry, the pumpability of which is not affected by the contamination of complex salts, such as tachyhydrite. The second system, Cement System 2, is a freshwater-based slurry used as a reference.

Results

A comparison of the performance of the two cement systems analyzed for the well conditions is made in terms of remaining capacity. It is a measure of the residual capacity of the cement sheath available for use, with a linear-elastic-behavior assumption. A remaining capacity of 100% corresponds to no variation compared with the initial situation (i.e., no loading); 0% corresponds to the onset of nonlinear behavior. Both shear and tensile modes of response should be compared because of the axial variation of creep rate. Because of the high resilience of Cement System 1, the remaining capacity is significantly higher than that of Cement System 2 at most sections.

It can be observed that salt/salt-­intercalation junctions play a critical role by subjecting the cement sheath to a combination of shear and tensile stresses. For example, tachyhydrite is the fastest-creeping salt. Therefore, that section of cement sheath adjacent to tachyhydrite layers will be pushed radially inward more than other sections. This will result in the cement sheath at the tachyhydrite/nontachyhydrite junction being subjected to tension. As a result, the tensile remaining capacity will reduce.

Because developing a design procedure is the central objective of this work, the analysis is performed and the results are compared for an optimum model size. It can be interesting to examine the parametric effects of different model sizes and salt-layering patterns. Regardless of model size, the general behavior will not change and Cement 1 will perform better than Cement 2.

Summary

The important aspects of the design procedure and the comparison exercise are as follows:

  • A systematic design procedure was established to estimate the cement-sheath mechanical integrity in the presence of intercalated creeping salts.
  • Obtaining correct salt-creep properties and accounting for overburden effects when using a realistic model size are two critical aspects for accurate predictions.
  • On the basis of the results of a finite-element model, elastic cement systems perform relatively better than conventional cement systems in the presence of both single and intercalated creeping salts.

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 26310, “Design Procedure for Cementing Intercalated Salt Zones,” by S.R.K. Jandhyala, SPE, and K. Ravi, SPE, Halliburton, and J. Anjos, Petrobras, prepared for the 2015 Offshore Technology Conference Brasil, Rio de Janeiro, 27–29 October. The paper has not been peer reviewed. Copyright 2015 Offshore Technology Conference. Reproduced by permission.