Shallow hazard detection and risk assessment through the acquisition and interpretation of Full Tensor Gravity Gradiometry in the Delaware Basin of Texas
Summary
Shallow hazards associated with dissolution karsting within the Castile formation of the Delaware basin of west Texas represent a significant drilling hazard. One operator encountered four karst related cavities within the first four hundred feet of section from two separate pad sites. The intersection of these dissolution cavities resulted in the loss of significant amounts of drilling fluids and concrete, resulting in the abandonment of the boreholes. Approximately 1100 line-km of Airborne Full Tensor Gradiometry (FTG) data are acquired at a nominal line spacing of 100m and drape elevation at 80m above the ground surface in order to rapidly identify and assess further risks to drilling in the adjacent lease acreage. 2.75D and 3D models are constructed to determine detectability and identify relevant wavelength and amplitudes based upon information collected from the karst cavities. Focused enhancements of the acquired data reveal a correlation to the drilled cavities, but the wavelengths are too broad to assess drilling risks. Curvature analysis of the FTG data reveals a striking correlation to cavern analysis performed in outcropping areas of the Castile formation. Riskiest and safest drilling sites are identified by means of analyzing the curvature information.
Full Tensor Gradiometry is rapidly acquired, processed and interpreted in order to assess shallow karst risk where cavities are expected in the top four hundred feet. Karst systems are identified and are consistent with mapped cavern systems in areas of Castile formation outcrop. If these cavern systems are open below the water table, new sources of water for drilling can be discovered.
Areas of both high and low risk are identified through curvature analysis. Assessment of the limits of detectability are underway to determine maximum depths and minimum cavern size in which the methodology is effective.