AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Improving Horizontal Well Placement and Completion Effectiveness in Deltaic Tight Sands - A Case Study in Anadarko Basin

Abstract

The Upper Pennsylvanian Cleveland tight sands in Anadarko basin consist of very fine grained and well-sorted sands that are interbedded with thin mudstones and siltstones. Reservoir sands are interpreted as tidally reworked fluvial deltaic systems, where clean sand bodies are mixed in with sand shale lamination sequences, which present a high degree of ambiguity when geosteering horizontal wells. In the past year, we have successfully drilled and completed more than two dozen horizontal wells in low porosity (3-15 p.u) and low permeability (4-400 uD) reservoirs. Subtle stratigraphic variations in these rocks have a profound impact on well productivity and ease of completion. Though the net sand package is 80-120 ft, the primary hydrocarbon producing rock type is only 8-30 ft thick. Low commodity prices demand low well costs and in general a basic MWD gamma ray (GR) tool is employed against offset vertical well coverage of variable quality. The task of landing and chasing ~10 ft thick sands along a 4000 ft lateral is daunting. Squeezing/stretching of MWD GR's to match the log character often results in a non-unique interpretation. We have observed inconsistencies in tool calibration among several MWD tools; an increasing separation between the true and MWD GR values was noticed with increase in shaliness. A calibrated GR log is essential in steering long laterals in these complex reservoirs. For this purpose, we acquired a whole core and a quad combo log data in one pilot hole along with triple combo logs on several wells. Further integration was done with high-resolution core GR and core analysis data to calibrate GR markers, redefine landing and steering targets improving real-time interpretation and drill wells with higher in zone percentages. Horizontal wells in thin laminated targets also present a challenge for hydraulic fracture initiation and optimization. We conducted detailed fluid sensitivity studies on cuttings from wells across the basin to optimize the fracturing fluid recipe minimizing any formation damage. Rock types with low clay volumes were found to be the least sensitive, highlighting the importance of steering the lateral in cleaner sand intervals. A calibrated pre-job geological model along with a systematic approach to support geosteering decisions and a tailored fluid chemistry for completion optimization was critical in overcoming challenges in these complex reservoirs.