Combating Gas Migration in the Michigan Basin
- D.L. Bour (Halliburton Services) | J.G. Wilkinson (Halliburton Services)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- March 1992
- Document Type
- Journal Paper
- 65 - 71
- 1992. Society of Petroleum Engineers
- 1.6 Drilling Operations, 2.2.3 Fluid Loss Control, 2 Well Completion, 1.14 Casing and Cementing, 1.14.3 Cement Formulation (Chemistry, Properties), 5.1.1 Exploration, Development, Structural Geology, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 4.3.1 Hydrates
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Gas migration has been a significant problem in cementing wells drilled in the Praire du Chien formation. Several types of gas-migration cementing systems have been used successfully during the past several years, but only recently has an analytical method been developed to help quantify the nature of gas-migration potential on an individual well basis. Once the probable limits of the gas flow have been established, it is possible to match an appropriate gas-migration-control cementing system for a specific set of well parameters. An important aspect of this process is computer analysis that establishes the probability of gas migration for a given set of well conditions and then selects an effective strategy for designing the cementing program for that particular set of conditions. This paper presents case histories illustrating applications of gas-migration-control cement systems with particular emphasis on the job design process.
Operators in the Michigan basin have encountered high-pressure gas formations during drilling and cementing the long intermediate interval before proceeding to drill to the Praire du Chien formation. While not considered economically viable producers, Zones A-1 and A-2 carbonate formations are highly pressured gas-bearing zones that have presented particular difficulties during drilling and cementing operations. When cementing the long intermediate stage containing Zones A-1 and A-2, operators have obtained good results with various gas-migration-control cement systems. Depending on the specific well conditions - formation pressure, job design, well geometry, etc. - an effective strategy for cementing the long intermediate interval may be formulated.
In addition to the gas-migration cement systems, operators also use other techniques, such as stage cementing, to reduce the risk of gas migration during cementing. Through careful analysis of these and other job factors, particularly cement-slurry design, the problem of gas migration can be reduced greatly. Computer cement job design simulators and design programs aid in the selection and evaluation process.
A major innovation in evaluating cement jobs for fields characterized by gas-migration problems was the development of a cement design program that provides cement-slurry design recommendations from the specific parameters for a given well. Examples of the gas-migration-control cement design program show how effective cement-slurry designs are formulated for Praire du Chien well completions. Factors that can cause gas migration in a cemented annulus and gas-migration-control additive types and their functions are discussed.
The mechanics and methods of preventing gas migration have been investigated rather intensely during the past 10 to 15 years.1 To solve gas migration, one must first understand how it occurs.
For gas to enter a cemented annulus, the hydrostatic pressure at the gas zone must drop to the pore pressure. Work done by Cooke et al.1 showed that the pressure in a cemented annulus begins to drop shortly after cement is pumped into place. Thorough investigation has shown that this pressure loss is caused by the combined development of volume losses, ?V, and static gel strength, S.2-3
Static gel strength provides a means of restricting the transmission of hydrostatic pressure in the annulus. The maximum pressure differential that can be supported (i.e., the static gel strength in a column) may be calculated with the following equation:
While a mechanism for restricting transmission of hydrostatic pressure is provided by static gel strength in the cement, actual pressure loss is caused by volume losses in the cement slurry.
Because pressure loss can be observed almost immediately after the cement is in place, the mechanism causing pressure loss must be operating subsequent to placement. Testing has shown that cement typically begins to develop static gel strength immediately after it stops moving. At this time, most of the volume loss that can occur in a cement slurry will be caused by fluid loss into permeable formations. Therefore, in most cases, the majority of volume loss can be attributed to fluid loss. Hydration volume losses typically are insignificant immediately after placement.
The maximum gel strength at which cement slurry is still fluid enough to allow gas to percolate through it is needed to calculate the severity of a potential gas flow problem and to develop logical solutions to gas migration. In work done by Sabins et al.,4 gas was found to percolate through a cement until it reached a static gel strength level of about 500 lbm/100 ft2 S. This value was chosen from this work as the maximum S of a cement slurry at which percolation of gas still could occur.
Knowing the maximum S at which gas flow can occur makes it possible to predict whether there is a potential for gas flow in a well. For instance, if a particular well has an overbalance pressure, po, of 500 psi when the cement is in place and a maximum pressure restriction, ?pmax, of 400 psi when the cement reaches 500-lbm/100-ft2 static gel strength, gas flow should not occur (Fig. 1). However, if ?pmax=1,000 psi and the po=500 psi, gas migration could occur. In this case, all the overbalance pressure could be lost before the cement slurry reached 500-lbm/100-ft2 static gel strength.
This method allows not only evaluation of whether gas migration might occur but also estimation of the severity of a potential gas-migration problem. This is shown below by taking the ratio of po to ?pmax to calculate a term defined as the flow potential factor, F.
If this ratio exceeds 1, then a potential for gas flow exists; the larger F, the higher the probability for gas migration to occur.
The solution to Eq. 3 is dependent on the variables from a particular well. The magnitude of F is used as an indicator for the type of gas-migration-control cement system required for effective control. For low F, a simple solution (such as fluid-loss control) may be sufficient, while higher values may require a more integrated approach to the problem.
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