Borehole gravity (BHG) surveys have been run in the Aquistore PTRC ESTEVAN OBS 5-6-2-8 CO2 sequestration observation well adjacent to the Boundary Dam coal fired power plant near Estevan, Saskatchewan. Aquistore an independent research and monitoring project managed by the Petroleum Technology Research Centre (PTRC) intends to demonstrate that storing carbon dioxide (CO2) deep underground into a sandstone formation, is a safe, workable solution to reduce greenhouse gases. These surveys were designed to both monitor the CO2 sequestration process and provide a test well for the latest version of the BluecapTM borehole gravity meter. This paper shows results of the borehole gravity log from August 2015. Presentation Date: Thursday, October 20, 2016 Start Time: 11:25:00 AM Location: 168 Presentation Type: ORAL

Abstract Borehole Gravity (BHGM) measurements are passive observations of the gravity field beneath the surface. The BHGM responds to the bulk density of a large rock volume surrounding the borehole, which is orders of magnitude larger than the volume sampled by nuclear logs or cores. A summary of the corrections applied to BHGM measurements during processing as well as simplistic rules-of-thumb for interpreting BHGM data in a uniform host rock are presented. However, processing and interpretation of BHGM data are complicated by changes in the density of formations intersected by the borehole, and modeling and inversion yield more sophisticated solutions. The first generation borehole gravity instrument was introduced in the 1960s and was suitable for large diameter petroleum wells. A second generation BHGM probe for mining and geotechnical applications was introduced in 2011. Using this recently developed probe several examples of BHGM data acquired in Canada and USA for mining exploration and CO2 sequestration are presented.

Abstract A new-generation borehole gravity meter (BHGM) and sonde have been developed and successfully operated under the rigors of a pressurized wireline logging operation in an Alaska North Slope solids injection well. The new BHGM addresses shortcomings of earlier technology, such as sonde diameter, maximum well deviation, depth correlation, reliability, and availability. Borehole gravity logging provides a large depth-of-investigation alternative to the very shallow formation bulk density measurement obtained with conventional nuclear density logging tools, and the measurement accuracy is close to that of a nuclear density log. Therefore, it is a candidate for density measurements in wells with large near-borehole effects such as invasion, mudcake, rugosity, and completion hardware that are detrimental to nuclear density logs. A BHGM is also well suited for reservoir injection applications, such as the present case study, where changes in the pore fluid, and therefore bulk density, are expected at large distances from the borehole (many tens of feet) and long times (months or years). We present the results of BHGM measurements made in a disposal well that has been receiving injected slurry of ground-up drilling cuttings and water for 12 years. Descriptions of the well injection history and BHGM log are presented along with performance data from the gravity and depth measurements. The BHGM density log showed agreement above the injection zone with a pre-injection openhole nuclear density log, but showed a large increase in density at the injection region. Software for 3D gravity forward modelling, constrained by the known total mass of injected material and known porosity, provided a reasonable spatial model of the injected material in the formation consistent with the gravity measurements. This type of analysis, coupled with time-lapse BHGM measurements, can also be used in reservoir monitoring applications to visualize movement of waterflood fronts or gas expansion zones before they reach producing wells.

Relative gravity meters are sensitive instruments capable of detecting small changes of the earth’s gravity field with a precision of a few parts per billion (10) over time scales of one second. They are often used to characterize earth-tides that vary with diurnal and semidiurnal periods. Recently, a superconducting gravity meter was successfully used to record low frequency gravest seismic modes ( 9) Sumatra-Andaman earthquake (Rosat et al., 2005; Ferreira et al., 2006). High frequency and high amplitude signals such as the S and P body waves and the Rayleigh and Love surface waves associated with earthquakes have traditionally been the purvey of seismometers. Seismometers are usually optimized to record seismic frequencies (0.1-10Hz) and are designed not to saturate during large amplitude signals. Gravity meters, on the other hand, are usually optimized to filter out seismic noise and often are too sensitive to faithfully record the high amplitude waves associated with the first arrival of an earthquake. Recently, these difficulties have been overcome with the introduction of a new type of gravity meter (gPhone) with both large dynamic range and high sensitivity.