Reservoir Development Using Offset VSP Techniques in the Denver-Julesburg Basin
Authors: Cramer P.W. 
Published: December 2007

Summary.
Multioffset vertical seismic profiles (VSP's) have been used successfully to delineate and develop a Sand D field in the Denver-Julesburg basin. Sand D production in Wattenberg field was established by the Sierra Energy and Berge Exploration Well 34–3. Subsequently, three successive wells failed to find the Sand D channel. A stratigraphic model study suggested that offset VSP techniques could be used to determine the extent of the Sand D reservoir accurately and to identify potential offset drilling locations. On the basis of this study, a five-offset VSP was conducted in the discovery well. Interpretation of the data produced an accurate map of the reservoir limits and identified two additional drilling locations. The first of these locations was drilled and completed in Sand D. Subsequently, a VSP was run in this well to complete the delineation of the reservoir and to reconfirm the second offset location.

Introduction
Efficient and economical development of reservoirs is a primary goal of production earth scientists and engineers. In small fields typical of many of today's discoveries, achieving this goal is becoming increasingly difficult because of declining prices and the small reserves involved. Channel-sand reservoirs are among the most difficult to develop efficiently, yet they frequently have excellent production characteristics. The difficulties arise from the depositional mechanisms involved in channels that create sand bodies whose thickness and quality can vary rapidly over short distances, perhaps as short as a few hundred feet. These rapid variations make it difficult to use conventional seismic data successfully for mapping. and the cost of three-dimensional (3D) surveys may not be justifiable. given the small size of the field. In such cases, VSP's can often be used as a cost-effective, accurate means to delineate reservoirs and to optimize the number and location of development wells. A case history is presented showing the successful development of a channel-sand field by use of VSP techniques.

Initial Field Development
Wattenberg field is located in the Denver-Julesburg basin in northeastern Colorado. A portion of the field stretches across acreage operated by Berge Exploration and Sierra Energy in Weld County, as shown in Fig. 1. A map of the acreage and a typical lithologic column for the area are shown in Fig. 2. Initial production on this acreage was gas from the Sand J series at depths of between 7,900 and 7,950 ft [2408 and 2423 m]. The third well (Well 34–3), however, also encountered a thick (26-ft [8-m]) Sand D section at 7,820 ft [2384 m], which was oil-bearing. As Plybon and Oldham described, Sand D is a member of the Dakota group and is believed to have been deposited in a channel system as part of a westward-prograding coastal-plain system. The productive channels can range in size up to 1,000 ft [300 m] across and as much as 40 ft [12 m] thick. Outside these channels, regional thickness is usually under 10 ft [3 m]. After Well 34–3 was completed, Wells 34–4, 34–5, and 34–6 were drilled (see Fig. 2 for locations), but none encountered the Sand D channel found in Well 34–3. Fig. 3 shows a cross section through the field at Sand D level. Following these unsuccessful wells, the operators decided that some form of seismic control was necessary to map the Sand D channel before trying another well. Key concerns were turnaround time for acquisition and processing, cost, and, of course, reliability of the data obtained. Turnaround time was important because of the desire to increase production from the Sand D reservoir as quickly as possible. Because conventional 2D seismics and a 3D survey were ruled out as a result of time and permitting factors, a VSP was selected as an alternative method to satisfy the rapid turnaround criterion and to provide high-resolution, detailed data for precise delineation of the reservoir.

Background on VSP Methods
The traditional VSP is acquired with a surface energy source placed close to the well location and a downhole tool containing a geophone package that is clamped to the borehole wall to record the seismic signals from the energy source. This procedure is repeated with the tool moved to a different depth in the well each time. Given small dips and a straight well, the information gathered comes from the immediate vicinity of the wellbore itself, and directly below the current total depth (TD) of the well. However, if the source is moved some distance away from the well, as in Fig. 4, information is collected along a lateral profile away from the well in the direction of the source location. By moving the source location or by simultaneously using more than one source at different locations, we can generate a series of lateral profiles to provide information away from the well in several different directions. In this fashion, then, multioffset VSP's can provide critical information about the quality and lateral extent of reservoir rock around a well. Offset VSP's have several inherent advantages over traditional surface seismic lines and 3D surveys. Chief among these is improved resolution, both laterally and vertically, because of the unique geometry of the VSP, which results in a much shorter energy travel path and therefore superior data quality. Further, VSP's can typically be acquired and processed in a matter of hours or a few days, instead of weeks or months for a 2D surface seismic grid or a 3D survey. Hence when time is critical, the offset VSP is a logical choice. Offset VSP's have two primary disadvantages. First, in complex structural situations, the proper acquisition and processing of VSP data can be quite complicated and may not provide the best information to solve problems related to the complex structure. Therefore, 3D surveys may be better suited to these cases. Second. the extent of the lateral profiles generated by offset VSP's is limited. In some cases, the information provided by offset VSP's may not extend over sufficient distance to be of optimum use.
JPT
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