Crosswell Seismic Applications in Tight Gas- Case Studies
Authors: Nalonnil Ajay , Marion Bruce P. , Dogra Sanjeev 
Published: May 2016

Abstract
Crosswell seismic provides high resolution reflection and velocity images of the subsurface at the reservoir scale between wells. The technique allows for better imaging of the structure and stratigraphy that may be obscured in conventional seismic applications. Crosswell acquisition employs tomographic surveying, in which a transmitter and receiver are deployed in separate wells with key seismic wavefields being recorded between the transmitter and receiver locations. With this approach, interwell velocity and reflectivity can be obtained from direct wave and reflection processing respectively.
Recent advances in the technology have shown that the application of crosswell seismic extends to unconventional gas development, in particular tight gas. In this paper, two case studies are discussed with the focus on high resolution imaging and integration of the crosswell technique with hydraulic fracture monitoring respectively. A brief description of the acquisition method, processing workflow and some recent advances in measurement integration are discussed.
These two case studies demonstrate that crosswell seismic is able to successfully image the inter-well geology and provide new method of monitoring the stimulated reservoir volume from hydraulic fracturing. The results clearly show that crosswell seismic can significantly reduce the uncertainty and risk associated with positioning new wells and efficiently deploying fracturing and injection processes.

Introduction
Traditional well log data provide high-resolution measurements of the formation very close to the wellbore. Surface-based methods provide greater spatial coverage but coarser resolution. Crosswell seismic provides images of the interwell space at reservoir scale, hence bridging the measurement gap between the existing scales (Figure 1).
The first example (Case 1) is from the the Pinedale Field, one of the largest gas fields in the Greater Green River Basin of southwest Wyoming, USA. Geologically it is a tight-gas sandstone reservoir in the Upper Cretaceous Lance Pool. The Pinedale Anticline has a 5000 foot thick gross hydrocarbon bearing section comprised of fluvial and flood-plain facies rocks deposited in a broad alluvial valley nearly coincident with an anticline structure.
Efforts to better understand the inter-well geometry of sand bodies and the efficiency of the hydraulic fracing techniques used to enhance production lead to the use of crosswell tomography and micro-seismic frac monitoring to map the orientation and geology between wells.
The second example (Case 2) is from the Vega area of the Piceance Basin, a prominent tight and coal seam gas province in Colorado, USA. The low permeabilities as well as great depths to reservoirs have called for hydraulic fracturing as one of the main gas production techniques in the area. In enhancing the hydraulic fracturing technique, a key question to be answered is fracture growth and how the injected fluids propagate away from the wells. Crosswell seismic in time lapse was acquired to help understand this process.