Abstract
In the search for unconventional shale plays with commercial potential, many operators have properties in petroprovince basins containing wells through potentially productive shale zones. These shales were often encountered as part of exploration or development programs for deeper conventional targets. Often, the overlying shale is known to have had gas or oil shows reported during initial drilling, but little or no additional geological data was acquired at the time.
This paper discusses the workflow and method to use the minimal information from these existing wells, and to quantitatively incorporate them into a basin exploration program. The process begins with a single new well, such as a sidetrack from an existing well, which is evaluated with the full array of open hole logging tools. Coring (conventional or sidewall), DFIT tests, and other shale-specific logging tools are performed on this initial well. Pre-existing wells that penetrate the objective shale can also be quantitatively assessed for relevant shale properties by using specialized logging tools, such as a combined through-casing pulsed neutron and sonic tool, to map relevant shale properties. These tools are calibrated to the open hole data to generate a wider distribution of data points containing critical shale properties that can be demonstrated to have a strong relationship with production.
After the data acquisition process has been performed, the data are combined with existing seismic and structural information to delineate the best areas for further evaluation. Using modern mapping tools, a basin can be rapidly appraised to identify sweet spots, providing further exploration targets for evaluation drilling.
This paper discusses limitations, best practices, workflows, and methods, and includes an example of a European shale evaluation log to demonstrate this exploration technique.
Introduction
Shale exploration often requires reviewing shale sections that overlie historically producing conventional reservoirs. These reservoirs, or even old exploration targets that did not become economically viable, often have wells penetrating the overlying shales. These shales may have organic rich mudstone sections that form potential targets for shale hydrocarbon production. They were often the trouble zones while drilling, exhibiting overpressure and gas shows.
These sections were often neglected when wells were logged. Either no data or very limited data was acquired in these old non-productive sections that now hold interest. The challenge for operators is in how to use existing information and assets to examine and appraise these zones without engaging in an expensive drilling campaign.
Given favorable circumstances, existing wells can be "refreshed,?? enabling a full suite of logs necessary for shale evaluation to be acquired. This means that only a single initial well must be drilled to calibrate the system (Pitcher et al. 2012), and existing wells that are at their end of life can be relogged using modern cased hole logging technology. If the well integrity is still viable, further testing may be preformed, such as a diagnostic formation integrity test (DFIT) (Craig et al. 2000), to assist in the evaluation of the asset.
After the data has been acquired from discrete wells, new mapping methods can be used to determine the most favorable areas to drill a pilot program. This incorporates all of the important data, and the mapping workflow enables the map to be constrained by more properties than demonstrated here. In the case shown, three discreet properties are conditionally mapped to highlight areas of collocated total organic carbon, porosity, and brittleness. If the seismic volume is added, then volumetrics can be added as a condition, further refining the map. Adding information regarding stress directions would enable the operator to plan the orientation of a horizontal well pilot program to appraise this property. This shortens the exploration cycle, provides greater insight into the shale prospect, and gives a firm basis for making appropriate appraisal decisions.
