Case Study: Hidden Fault Detection in Haynesville Shale Prevents Losses of 33% Gas Production

Vertical seismic profiles allows operator to properly guide boreholes saving USD 8M in rig costs

Challenge: Determine correct structural interpretation of the Haynesville shale between two pilot holes showing a 500-ft depth difference in tops.

Solution: Image the structure between two exploration wells by designing, processing, modeling, and interpreting two walkaway borehole seismic surveys and a zero-offset vertical seismic profile (VSP).

Result: Discovered a fault between the wells and revised horizontal well plans accordingly to remain within target zone; avoided potential loss of USD 8 million in well costs and 33% gas production.

Planning in the presence of structural uncertainty

The organic-rich shale of the US Gulf Coast is now the biggest natural gas play in the United States. With ultimate recoveries estimated at more than 250 Tcf, it is one of the largest gas fields in the world.

An operator planned to drill two 5,000-ft horizontal wells into the Haynesville of Louisiana from nearly vertical pilot holes about 11,000 ft apart. Original well plans called for drilling the pilot wells, acquiring openhole logs, and evaluating the well tie, then drilling the horizontal wellbores—one south of the north pilot, the other north of the south pilot.

Correlation of formation tops showed that the Haynesville was about 500 ft deeper in the north pilot than in the south pilot. No surface seismic data were available. In the absence of additional data, the operator’s initial drilling plan assumed a constant 3° structural dip between the two wells. However, an incorrect interpretation of this shift in depth could threaten the success of the horizontal boreholes. Drilling out of zone would reduce the ultimate stimulated reservoir volume and impact both well costs and production.

Processing and designing borehole seismic surveys

After consulting with the operator on its requirements, Schlumberger proposed running two walkaway VSP borehole imaging surveys to image and delineate the Haynesville structure between the two wells. This would enable drilling engineers to properly guide the 5,000-ft horizontal boreholes.

Schlumberger specialists carried out detailed prejob planning to ensure proper survey design. The shale-carbonate boundary between the Haynesville and the underlying Smackover field, which produces a strong seismic reflection, was the designated target. Using three vibrator trucks as the source array, Schlumberger used a large seismic array with three-component geophones to acquire a 20,000-ft primary walkaway line through both wells, shorter secondary lines through each surface location, a zero-offset source location in the north pilot, and an offset position west of the south pilot.

Experienced analysts ran the raw field data through a sophisticated processing, velocity modeling, depth migration, and imaging workflow. Using well logs and zero-offset VSP data, they tied the Smackover top to the regional borehole seismic image. Results were presented to the customer, with recommendations based on improved understanding of the interwell structure.

Resolving structural uncertainty, ensuring drilling success

Expert depth imaging and interpretation of the walkaway borehole seismic surveys revealed that the Haynesville/Smackover package was generally flat, rather than dipping at 3° as originally assumed. Petrotechnical experts identified a southward-dipping fault approximately 4,000 ft north of the south pilot, which explained the 500-ft difference in the depths of formation tops.

Well plans were revised accordingly to specify the drilling of both horizontal wellbores at 90°, with the option to drop angle in real time, if necessary. The fault was encountered where specialists had predicted. Both horizontal sections successfully remained within the Haynesville shale and stopped before crossing the fault. Had the operator proceeded with its original plan, about one-third of each wellbore would have exited the target zone. The operator would have potentially lost USD 8 million in well costs and 33% of its gas production.

Both wells were completed and hydraulically fractured, yielding production rates that “pleasantly surprised” the customer. Initial production was about 5,500 Mcf/d from one well and 6,700 Mcf/d from the other.

Download: Hidden Fault Detection in Haynesville Shale Prevents Loss of 33% Gas Production (1.17 MB PDF)

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Improving Drilling Decisions
with Seismic Surveys

In the absence of surface seismic data, correlation of well logs indicated a 500-ft differenceRunning raw field data from the walkaway borehole seismic surveysBorehole seismic data revealed a previously undetected fault between the wells
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