A Case Study of the Upper Cotton Valley in the Oak Hill Field of East Texas
Authors: Richardson Michael F. 
Published: December 2007

This paper summarizes a study of Upper Cotton Valley completions in the northern half of the Oak Hill Field in the East Texas Basin. The study involved the evaluation of full core, sidewall core, special permeability measurements, x-ray diffraction, thin sections, SEM, conventional open hole logs, magnetic resonance imaging tools, micro-resistivity imaging tools, production logs, build-up data, and forty-eight Upper Cotton Valley completions.

The overall goal of the project was to improve the completion economics of the Upper Cotton Valley in Oak Hill. This paper details improvements made towards the identification and prediction of productive Upper Cotton Valley sands. An in-depth analysis of the stimulation and completion procedures for the Upper Cotton Valley was also undertaken. That portion of the Upper Cotton Valley study primarily involved the development of a new low sand concentration fracture stimulation technique. These results are presented in a separate paper, SPE 38611.

In this paper, a method is described that could be applied to the development of other stacked tight gas sand reservoirs where conventional open hole logs or other conventional evaluation methods have proven to be unreliable. The method resulted in a 50% reduction in average completion costs, with no loss in average initial production rates or reserves per Upper Cotton Valley completion.

Introduction

As Fig. 1 indicates, Oak Hill Field straddles Gregg, Rusk, and Harrison counties within the East Texas Basin. The primary producing interval in Oak Hill is the Cotton Valley Sand of Upper Jurassic Age which, to date, has produced an estimated 840 bcf from the field. Significant production has also been obtained from the Pettet and Travis Peak formations.

In this area of the East Texas Basin, the Cotton Valley Sand is a 1,300 to 1,500 ft. thick section of alternating sand and shale. Traditionally, the Cotton Valley Sand in Oak Hill has been separated into the Upper Cotton Valley (UCV) and the Taylor intervals. The Taylor is the deeper interval and is 250' to 300' thick. The Taylor is a barrier bar complex with a northeast to southwest orientation that corresponds to the perceived paleo shoreline, the identified fracture orientation and the production trend. The Taylor interval is fairly continuous and can be correlated throughout the Oak Hill Field. The Upper Cotton Valley makes up the remainder of the 1,300–1,500' of Cotton Valley Sand and in this portion of the East Texas Basin, it is believed to be dominated by fluvial deposits. Therefore, the Upper Cotton Valley is much less continuous than the Taylor, and in most cases, individual sands cannot be correlated between wells on 80 or even 40 acre spacing.

Porosities and water saturations determined from open hole logs are generally very similar between the Upper Cotton Valley and the Taylor, averaging about 8% and 35%, respectively. The rocks have all undergone a significant amount of diagenesis that has almost completely occluded the primary pore space with calcite cement and reduced average permeabilities to less than 0.01 md.

Historically the wells have been completed by isolating and stimulating the Taylor interval with a single massive hydraulic fracture. The Upper Cotton Valley has then typically been fracture stimulated in three to five stages depending upon the amount of sand present and the location and presence of shale barriers. Production from all stages has then been commingled in the wellbore and measured as a single stream.

Justification for Study

In the past, there has been little effort to quantify the individual contributions of each of the stages and, therefore, justify their expense. Some of the major reasons why this analysis had not been performed are as follows:Production from an individual stage or sand package is typically commingled and measured as a single stream.

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