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AAPG Southwest Section Annual Convention

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Managing the Risk of Shallow Drilling Losses in the Delaware Basin


Shallow drilling losses are a significant problem in the Delaware Basin because of the presence of subsurface karst features. Karst processes produce voids and cavern systems that result in drilling losses. Airborne full tensor gradiometry (FTG) measures the directional components of the gravity field. Multiple simultaneously acquired tensor components allow identification of anomalies associated with subsurface voids. Pad-specific seismic employs very dense surface seismic over potential well locations, using routine techniques except for the increased density of geophone and seismic sources. Both techniques were evaluated as predictive measurements. For FTG, negative gravity anomalies are highly correlated with drilled cavern network geometry and fracture networks. The most negative portions of the negative maximum along axial traces identify the highest risk for karst penetration. Further refinement improved the method’s prediction of caves. For pad-specific seismic, feasibility tests showed good promise and were followed up with a field survey over a pad with known drilling losses. The results showed good imaging of the surprisingly large feature which had taken the drilling fluid. This is the first application of FTG to classify drilling risk of karst features in the Permian basin. The FTG hazard map provides operational integrity of surface location selection and replaces surface topography techniques. The FTG data and analysis also hold value for fault mapping and for water drilling efforts in the area. Similarly, pad-specific seismic results were a valid solution. Both methods are solutions to avoid shallow drilling risks from karsting in the Delaware Basin and, as co- measurements of same features, together provide synergies in multiple ways.