Development-Well and Execution Planning in a Complex Subsurface Environment - The Effect of Large- and Small-Scale Modeling on Well Deliverability
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 101874, "Development-Well and Execution Planning in a Complex Subsurface Environment - The Impact of Large- and Small-Scale Modeling on Well Deliverability," by G. Philip, SPE, S.K. Twartz, SPE, and J. DePledge, Woodside Energy, prepared for the 2006 SPE Asia Pacific Oil & Gas Conference and Exhibition, Adelaide, Australia, 11–13 September.
The Thylacine gas field is in the Otway basin (offshore southeast Australia). The interplay of intrafield faulting and the layered geology provide potential for a high degree of compartmentalization. A significant proportion of the reserves is thought to reside in reservoirs of the Thylacine sandstone member, which contains poor-quality marine sands interbedded with successions of shale and thinly bedded sand/shale. Key to designing wells to achieve satisfactory deliverability was an understanding of the distribution of net reservoir in the background of nonreservoir.

Introduction
The Thylacine field is 70 km offshore Victoria in Tasmanian waters. The field was discovered and appraised in 2001 with Wells Thylacine-1 and Thylacine-2 (Fig. 1), which penetrated gross hydrocarbon columns of 275 and 240 m, respectively, in the Waarre and Flaxman marine-deltaic sandstone formations. Overlying Belfast shales provide the seal to the accumulation.
The discovery and appraisal wells indicated that productivity would be low from the upper reservoir units and that considerable reservoir distribution and connectivity uncertainty exists. These elements present significant challenges to the commercial development of this resource. Any well drilled into sections having lower net-to-gross reservoir-thickness ratio must be placed carefully to enable commercial deliverability. Therefore, a detailed predictive reservoir stratigraphic model integrated with seismic inversion was used. The model increased confidence in the location of the target reservoir.

Development Challenges
A combination of complex field structure, reservoir-facies areal distribution, and vertical-stacking patterns presented considerable challenges to developing the Thylacine field. Low-quality potentially compartmentalized reservoir sands in Thylacine hold significant gas reserves. Wells in this environment must be designed to intersect sufficient productive reservoir to maximize productivity and connect potential pressure compartments at commercially viable levels. Uncertainty related to reservoir-facies development and the effect of small-scale faulting on individual-well productivity also meant that final horizontal lengths of wells could not be preset and had to be determined as the reservoir section was drilled. Detailed reservoir modeling across the range of potential subsurface outcomes before drilling, translating net-to-gross reservoir-thickness ratio to a productivity and value measure by use of dynamic simulation, provided a working tool for determining optimum horizontal-well length during drilling.