Analysis of Devonian shale production histories without careful data screening can easily mislead even the most exacting investigator. Trends established from cumulative production data may not accurately represent the true production pattern. The difference between a good well and a poorer producer may be the result of fracture depletion or the length of actual producing time (among other factors) rather than a change in reservoir quality. The use of matrix controlled production information, derived from decline curves, appears to resolve this problem.
Columbia Gas is performing a study aimed at identifying the mechanisms controlling Devonian shale productivity. To accomplish this Columbia is productivity. To accomplish this Columbia is evaluating relationships among geological setting, reservoir parameters, production data and man-made influences (e.g. stimulation). Approaches to the study consist of a production pattern analysis and an individual well study. The former concerns a large scale regional examination of production data. Hopefully, a correlation will exist between production trends and mappable tectonic and/or depositional production trends and mappable tectonic and/or depositional features. The individual well study/deals with the identification of specific permeable features controlling productivity in a given well.
This paper presents preliminary information regarding Devonian shale production data as it applies to this study. Specifically, this paper will:
look at criteria for classifying wells as Devonian shale producers;
identify factors which directly influence productivity;
show how different interpretations can result from the use of various types of production data; and
suggest a method for selecting production data used in establishing production patterns.
The concept developed in this paper should result in reliable identification of producing trends. This idea evolved very early in the study. Future refinement of this approach and its application are anticipated as this Gas Research Institute sponsored project continues. project continues.
The first step in evaluating shale data involves determining whether the wells in fact produce from the shale. Many "shale" wells contain other horizons which contribute to production. The unwitting inclusion of that data will result in a production bias. That is, a "shale" well, good or poor, would mistakenly appear better because of nonshale production. The use of such information may introduce production. The use of such information may introduce significant errors into the identification of apparent production trends; it could also adversely affect the individual well study selections and subsequent analyses. To minimize potential problems, it becomes necessary to identify a well as exclusively producing from the shale or to qualify it as a shale producer.
Classification of a well as a shale producer results if the method of completion exposes only the shale horizon to the wellbore. Figure 1 depicts several examples of shale producers. The wellbore on the left shows all horizons cased off and isolated from each other by cement; shale production enters the wellbore through perforations. The middle sketch depicts an open hole shale completion. Cemented casing partially penetrating the shale isolates other horizons from the shale. The right diagram shows another zone explosed to the wellbore. However, a packer isolates the shale production from the nonshale interval. In the situations described, the assumptions include: no communication between the shale and other formations via natural or induced fractures; no channeling behind casing due to poor cement bond; or no casing, tubing, and packer leaks.
The wellbore configurations in Figure 1 represent a few desired geometries for shale well classification. However, many shale wells contain other potentially productive formations exposed to the potentially productive formations exposed to the wellbore (Figure 2).