Low-impact seismic acquisition illuminates oil shale

The geophysical community is adjusting to new conditions in onshore geophysical operations inlcuding a larger share of unconventional resource targets, higher resolution requirements, more environmental sensitivity in operations, continued improvement in health and safety, and ever-increasing cost challenges. Shell Oil's seismic acquisition method for Piceance basin oil shale targets was developed with these challenges in mind.

The Piceance basin, which sits in a rugged, mountainous area of northwestern Colorado, has the richest oil shale deposits in the world, with more than 2 Bbbl in place per square mile in the central basin area. In the central portion of the basin, the oil shale beds contain nahcolite, a form of evaporite. This deposit is important to characterize because it can impact fluid flow and energy efficiency. Shell scientists felt seismic could offer a tool for nahcolite delineation.

The full weight-drop device weighs 70 lb. (Images courtesy of Shell Oil)

The study area in the Piceance basin sits in an area of limited 2-D seismic coverage. The team opted to shoot a limited high-resolution 2-D pilot survey to ascertain seismic feasibility. Results demonstrated significant upgrade in data quality and clear ability to pick nahcolite boundaries.

With the value of high-resolution seismic proven, the team initiated an effort to create a practical means of acquiring high-resolution cross-country 3-D seismic since high-resolution deep-hole acquisition was prohibitively expensive. Data obtained in the pilot acquisition indicated potential utility for a lower energy source.

Weight-drop system

After studying numerous low-impact sources, the team landed on the idea of a man-portable weight-drop system. While weight drops are an old concept, it is actually a challenge to design light-weight, portable, and safe weight-drop devices. After many iterations, a significant number of improvements were incorporated into designs. In the latest designs, the device is a 3-m to 3.6-m (10-ft to 12-ft) PVC pipe mounted on a standing frame. The top of the PVC pipe has a pulley through which a rope pulls a 25-lb weight. The operator pulls the weight to the top and lets it fall to a baseplate at the bottom.

The ultradense freezewall tomography test was able to observe changes in rock velocity during freezing.

The system, operated by a three- or four-man crew, is completely portable. Each end of the device is carried by a crew member while the baseplate is carried by a third crew member.

Key elements of the device are that the weight falls from the same height every time (regardless of operator) and that the use of a pulley allows the operator to stand upright and comfortably raise the weight with minimal back stress. The weight is not exposed at any time except for a couple of inches at the bottom of the PVC pipe, so it is very safe, especially when compared to operating dynamite drill-holes delivered by helicopter. Also, outside of the imprint of the baseplate, there is no environmental impact – it is readily permitted.

The weight-drop test line is compared to nearby vibe high-resolution lines.

This is a difficult task when considering conventional cabled systems. However, cableless devices provide a means to lower the weight per receiver enough to render walk-in delivery possible. Calculations and initial tests indicated the combination of man-portable weight-drop/cableless recording was not only feasible but also inexpensive enough to enable high-resolution 3-D acquisition. The team decided to conduct another field test.

Mobilization showed the many advantages of the weight-drop/cableless combination. No instrument truck, helicopter, or jug trucks were required. The weight-drop devices were constructed locally. The cableless boxes arrived via two pallets delivered by UPS. The crew was small, only 33 men (which included a full complement of HSE oversight), and all transportation of boxes and manpower was in vans or small trucks.

The entire operation took six days including mobilization/de-mobilization, HSE training, geophone deployment, and recording. Deployment of the geophones and boxes was smooth since the equipment was light enough to carry in via backpack despite treks up and down steep inclines. Antennae relays for communication of GPS timing signals to the boxes were reliable across the entire line. No change of batteries for receiver boxes was required during the operation.

The weight drop operation was initially slow, but production steadily increased with a maximum of more than 700 source points in one day. This figure was obtained by using four simultaneous weight-drop acquisition teams.

HSE evaluation indicated this operation was safer than similar operations of standard crews. The combination of cross-country hiking and weight-drop operation at high elevations did not over-exert the crew. There were no recordable injuries. The test provided encouragement that low-impact acquisition could be performed at low cost and at low HSE exposure relative to traditional methods. For processing, the very high spatial sampling provided superior results for refraction statics, linear noise removal, and migration. Data obtained via this type of acquisition at oil shale depths are nearly as high-quality as those obtained via stronger sources.

Given results from the initial tests, a follow-up experiment at Shell's Freezewall pilot was undertaken. This was a tomographic experiment to evaluate properties in the vicinity of the freezewall. The weight-drop source was deployed at a very fine scale at the surface and recorded into several downhole geophone deployments. The experiment was a success in measuring tomographic arrival times and obtained observable changes in rock velocity under the process of freezing. This experiment could not have been performed with dynamite or vibroseis in the very tight confines of the operational environment.

Finished processing exhibits oil shale data quality comparable with viobroseis acquisition. However, it must be stressed that the weight-drop system is a weak source. Under no circumstances will the data quality data exceed that of stronger sources with identical acquisition geometries. For now, a decision to use a weight-drop/cableless system should be based on trade-offs in data quality for lowered cost, lowered safety risk, and lower permit restrictions. This trade-off is compelling for shallow objectives such as oil shale.

Acknowledgements

Key support and contributions were made by Shell geophysicists Jorge Lopez and Patsy Jorgensen. The authors wish to thank primary contractor RII under direction of Harold Merry and Jim Roy for continued assistance in development of the man-portable source. Other suppliers also were instrumental to results, including Ascend-Geo and Oyo-Geospace. This paper is based on a presentation given at the 2011 Society of Exploration Geophysicists annual meeting and has been reprinted with permission from the authors.