325880 Ensuring Environmental Friendliness of Horizontal Shale Gas Wells Through Zonal Isolation: A Model-Based Approach

Wednesday, November 6, 2013: 3:35 PM
Union Square 11 (Hilton)
Shyam Panjwani, Chemical & Biomolecular Engineering, University of Houston, Houston, TX and M. Nikolaou, Chemical and Biomolecular Engineering, University of Houston, Houston, TX

Shale gas horizontal wells face problems related to gas leakage from various zones of a well into the air and water reserves. Protecting the environment and improving well productivity is one of the biggest challenges for shale gas production. This problem can be solved by improving zonal isolation. Zonal isolation is necessary to avoid any communication between rock formations penetrated by a well. Zonal isolation mainly depends on the quality of well cementing, namely the tightness of the seal created by the cement placed between a metal casing and well walls.

Cement bond quality is governed by various physical factors and conditions. Casing data (internal casing diameter, number of centralizers and casing-hole relationship), cement system data (slurry type, density, yield, additives composition), rheological properties of drilling mud & cement and operating temperature and pressure conditions. Cement bond quality is measured in terms of a bond index (BI) value, with zero value indicating poor cementing and value at 1 representing perfect cementing.  The BI value can be calculated using Cement Bond Log (CBL) data, following a method given by (Fitzgerald 1985).

As it has been well established that a large number of physical factors affect the overall cement bond quality and corresponding BI value, a model capturing the effect of such factors on BI would be useful for the design of effective and efficient cementing jobs.  Such a model can be developed from available field data.  Because data are relatively scant, Partial Least Square Regression analysis (PLS) (De Jong 1993) is used for model development.

A PLS model was built using as inputs dimensionless quantities related to all physical factors considered to affect BI, and as outputs the average and standard deviation of BI along the length of a cemented well casing. It was found that out of all quantities taken in the analysis, few dimensionless quantities played a major role in affecting the BI.  Cross-validation (Leave-one-out) results obtained from the PLS model are presented in Figure 1. Figure 2 shows the latent variable loadings for major contributing dimensionless factors.

Figure 1  Cross validation VS Experimental BI data

Figure 2  Latent variable loadings for various dimensionless quantities

The above as well as additional results on the average and standard deviation of the BI are discussed in the presentation.

In conclusion, the PLS model presented here can be used to qualitatively determine the significance of physical factors affecting the quality of well cementing (captured by BI value).  More importantly,  it points to the need for additional data such that model-based tools can be built that will aid in ensuring consistent and predictable quality of well cementing jobs. References

De Jong, S. (1993). "SIMPLS: An alternative approach to partial least squares regression." Chemometrics and Intelligent Laboratory Systems 18(3): 251-253.

           

Fitzgerald, D. D., B.F. McGhee, and J.A. McGuire (1985). "GUIDELINES FOR 90% ACCURACY IN ZONE-ISOLATION DECISIONS." Journal of Petroleum Technology 37(12): 2013-2022

               

 


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