High-Resolution Seismic Reflection to Monitor Change

High-resolution seismic reflection has proven a valuable tool detecting changes in fluid composition, rock petrophysical properties, and structures critical to reservoir production management and groundwater protection in Kansas. Surface seismic reflection is not a method that lends itself to direct detection and delineation of boundaries between different fluid compositions in porous media. However, time-lapse seismic does appear to have been successful identifying areas where calculated changes in seismic characteristics (specifically velocity) are greater than 10% at a miscible CO2 flood in Russell County, Kansas. Empirically a 10% change in seismic velocity has proven to be the minimum practical threshold where signal emerging from the noise can be interpreted with any degree of confidence. This change in velocity occurs when the saturation of injection CO2 exceeds 30% of the total pore fluid at this site. To evaluate the potential of high-resolution seismic reflection to monitor the injection in a miscible CO2 enhanced oil recovery pilot study in a 900 m deep 5 m thick oolitic carbonate petroleum reservoir, a 4-D seismic reflection program was undertaken that includes 12 different 3-D surveys over 6 years. The first 3 years (8 surveys) were designed to specifically address the potential application of this method to enhanced oil recovery. The last 3 years (3 surveys) are intended to evaluate the effective of seismic in providing the assurances necessary for CO2 sequestration. Collapse structures related to karst features and anthropogenic leaching resulting from faulty bore fluid containment have posed serious threats to the quality of groundwater above the Hutchinson Salt Member of the Permian Wellington Formation in central Kansas. High-resolution seismic reflection played a key role in characterizing the preferential growth of a sinkhole resulting from the dissolution of the Hutchinson Salt in Pawnee County, Kansas. Salt leaching was instigated by confinement failure of an oil field brine disposal well. In 1998, legacy 2-D seismic data showed the subsurface extent of collapse was approximately an order of magnitude larger than the sinkhole. A consistent pattern of growth, elongated parallel to the anticlinal structure responsible for the oil field, was interpreted on 2004 time-lapse 2-D data. Confinement of several aquifers overlying the salt was compromised when the 300 m of rocks overlying the salt collapsed, forming the sinkhole. This breach in confining layers provided a pathway to the salt for unsaturated brine fluids. Radial growth of the dissolution feature has slowed consistent with volumetric spreading of the dissolution front. The migration of the brine away from the dissolution front and out of the Hutchinson Salt interval has been relatively consistent in spite of changes in source waters. High-resolution seismic monitoring has a great deal of potential to monitor changes in fluid and structures, but requires a high degree of scrutiny and attention to detail for effective application.