Parametric Study of Gas Entry Into Cemented Wellbores
- Fred Sabins (Westport Technology Center Intl.) | M.L. Wiggins (U. of Oklahoma)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 1997
- Document Type
- Journal Paper
- 180 - 187
- 1997. Society of Petroleum Engineers
- 1.11 Drilling Fluids and Materials, 1.14.3 Cement Formulation (Chemistry, Properties), 5.1.1 Exploration, Development, Structural Geology, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 2.2.3 Fluid Loss Control, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.6 Drilling Operations, 1.14 Casing and Cementing, 4.3.1 Hydrates, 4.2.3 Materials and Corrosion, 5.2 Reservoir Fluid Dynamics
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A cement slurry is placed in a wellbore to harden into an impermeable mass that seals the annulus from fluid flow and protects the casing from corrosion for the life of the well. If fluid flow does occur in the form of gas migration, expensive remedial squeeze-cementing techniques are generally required.
The objective of the work covered in this paper was to study the parameters that affect entry of gas into a cemented annulus. This research incorporated a detailed study of the factors that contribute to gas influx from the time of initial placement of the slurry, through the gelation or transition state of the slurry, to the set condition. On the basis of the understanding of the processes involved in gas entry, a simulator was developed that predicts the amount of gas that enters a cemented wellbore, and identifies the critical parameters that affect the gas entry.
This study concerns itself with a portion of the gas flow problem: the entry of gas into a cemented annulus - and not with the flow of the gas up through the cement and the formation of a gas channel.
This study will provide insight to the following questions.
What cement properties are important to minimize gas entry?
What role does fluid loss play in minimizing gas influx?
What well parameters affect gas entry?
A computer simulator was developed for establishing detailed parameters that affect gas entry. Specifically, this simulator used mathematical relationships to describe the chemical and physical processes involved in the gelling and setting of the cement column. When the cement is placed into the annular space, the cement will lose hydrostatic pressure until the pressure in the annular column equals the formation pressure. At that time, fluid will enter the annulus based upon the volume losses in the wellbore. This research approached the problem by mathematically modeling the gelling and setting of the cement, as well as the hydrostatic-pressure loss and gas entry in the cement column.
The hydrostatic-pressure loss and ultimate gas entry in a cement column were studied by investigation of three cement-slurry properties and three well parameters. The cement-slurry properties addressed were static gel strength, volume losses, and permeability of the gelling cement. The static gel strength, or shear resistance, sets the maximum limit on the pressure decrease that can occur in a cement column; the volume losses in the slurry permit the potential pressure loss. Volume losses can occur as a result of two mechanisms: fluid loss from the cement slurry and hydration volume loss. The permeability of the cement enables volume losses from the wellbore to be transmitted through the entire cement column. The interstitial water in the matrix of the gelling cement can experience Darcy flow when a differential pressure is applied. The relationship between static gel strength, volume losses, and permeability are addressed in detail. The well parameters studied are geometries, length of cement column, and overbalance pressure. A parametric study was performed to determine which parameters are important in dealing with gas entry.
This parametric study involved developing detailed equations for three principal aspects of the gas-flow process: the physical properties of the cement during the gelation-and-set period; the pressure loss in a column of cement, taking into account the slurry properties and the well parameters; and the influx of gas into the cement column.
Physical Properties of Cement
The loss of hydrostatic pressure within the annular column must occur before gas can enter. In this study, it was assumed that the hydrostatic pressure within the annular column exceeded the reservoir pressure immediately after placement. Once the cement started to gel, volume changes occurred; the hydrostatic pressure in the annular column decreased as a function of the basic characteristics of the cement slurry as it gelled and sets. Equations were developed for four key physical properties that affect the hydrostatic-pressure loss in the column of cement: static-gel-strength development, hydration volume reduction, fluid loss to permeable formations, and permeability of the gelling cement.
Slurry designs that try to address the issue of gas migration generally focus on one or more of the above properties. Some additives used in slurries modifiy the static gel strength by using delayed gelling or thixotropic characteristics. Most all of the gas migration materials effect the fluid loss of the cement. In addition, some latex cements are sold as low-permeability cements. Each of these parameters will be described individually by mathematical equations; however, they are related to each other in the wellbore and must be studied collectively.
Static Gel Strength.
Static gel strength is a resistant force applied to the wetted perimeter of the surface of the borehole and the pipe. Static gel strength is the result of a small amount of hydration products being formed in the early stages of hydration. The values of importance are those up to the initial set of the cement slurry. This initial set of a cement slurry is the point at which the cement has some solid character and is generally defined as 50-psi compressive strength.
Compressive strength refers to the capability of the cement to support a compressive load (measured in force per unit area). Compressive strength is the most common property used to describe the solid characteristics of a hydrating cement. At some point in the cement-hydration process, the cement slurry will have enough solid character that the gas will not percolate up through the cement column. At what gel strength this solidification will happen is unknown. It is assumed that a cement slurry obtaining an initial set would prevent any additional gas from forming a channel in the matrix of the cement.
The only data available that relates static gel strength and compressive strength for two cement slurries are shown in Table 1. This table shows two different cement slurries having a compressive strength of 250 and 200 psi, respectively, at the equivalent static gel strength of 2,400 lbf/100 ft2 gel strength. Although this work contains only two data points, a reasonable estimation of the initial set value of static gel strength can be made for the purpose of this study. From an extrapolation of the data in Table 1, it was assumed that a static gel strength of 2,000 lbf/100 ft2 would be required to achieve the 50-psi compressive-strength/initial-set value. This value was used as the upper limit on the time for the analysis of the gas influx into a cement column.
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