Performance and Analysis of a Borehole Gravity Log in an Alaska North Slope Grind-and-Inject Well

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 groundup 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. IntroductionThe Prudhoe Bay field, Kuparuk River field, and other major oil fields are located on the northern coast of Alaska, approximately 300 miles above the Arctic Circle. Development wells were directionally drilled from gravel well pads. Drilling waste comprising mud and cuttings was originally stored in reserve pits, actually impoundment areas surrounded by areas of the pad and access roadways. The grind-and-inject (G&I) project was initiated to dispose of this stored waste to allow closure of the reserve pits. Today, new drilling waste and some production waste is also disposed of at the G&I plant.Waste is excavated during the winter and hauled to the G&I plant. It enters the plant by conveyor and is mixed with heated seawater inside a rotating ball mill. The ball mill reduces the mix to slurry which can be injected into one of three UIC Class I disposal wells. Injection is cycled among the three wells with a typical injection period in a single well of 1 to 2 weeks. In July 2010, approximately 124 million barr...