THE MARCELLUS SHALE: A MAJOR UNCONVENTIONAL ONSHORE GAS PLAY IN NE USA
Lois Atterton2, Celine Wise1, lain Scotchman3, Ger van Graas2
1Statoil Houston, 2Statoil ASA, 3Statoil UK
In mid-November 2008 Statoil completed a US$ 3.3billion Joint Venture deal with Chesapeake Energy, gaining the company an entry into a new form of unconventional hydrocarbon E & P: onshore shale gas. The deal gave Statoil a 32.5% stake in some 1.8 million acres of CHK’s leases in the NE USA. The shale gas is sourced, reservoired and trapped within the Marcellus Shale located in the Appalachian foreland basin in the states of Pennsylvania, West Virginia and New York, covering an area of some 31 million acres.
The Marcellus Shale is a middle Devonian-aged organic-rich petroleum source rock with TOC up to 12% which sourced much of the conventional hydrocarbons produced from the area over the last 150 years. While maturity ranges from oil window to post-mature, the shale is gas mature over much of the area with optimum maturity for shale gas production ranging from 1.0 to ~3% Ro. The shale ranges from around 50ft thick in the SW of the play in West Virginia, thickening into the depocentre in NE Pennsylvania where it is over 600 ft thick. In the organic-rich layers which form the best reservoir, porosity ranges from 4 – 6% with permeability of 100 – 1000 nano darcies. The gas is trapped within the shale source rock both as free gas within the intergranular and intra-kerogen porosity and as adsorbed gas attached to kerogen particles and clay minerals. GIP ranges from 60 to 160 BCF/square mile and the shales lie at depths of 4000 to around 9000 feet.
Gas production is achieved by the drilling of a large number of horizontal well-bores up to 6000 ft long, generally with 6 - 8 horizontal wells being drilled from each well pad. Multi-stage fraccing using using large volumes of water and sand proppant is required to release the gas from the rock. Initial large flows of gas (up to 10mmcf/day) rapidly decline as the free gas trapped in the shale pores is depleted, being replaced by the slow release of de-sorbed gas as pressure decreases. This gives each well a potential productive life of 30 – 60 years. Ultimate production is currently estimated at 3 to 8 BCF per well.
To be successful, the play requires the continual drilling of new wells to replace those on the decline. 3-D seismic and micro-seismic are becoming increasingly important in the locating of wells, avoidance of geo-hazards and monitoring fraccing. Improved drilling techniques combined with longer horizontal bores and optimised frac techniques both reduce the cost of individual wells while increasing their productivity.