Abstract

A mechanical earth model (MEM) and wellbore stability analysis were completed for a CO2 injection well in the Eau Claire shale where in offset wells, severe borehole enlargements were experienced. This shale serves as a caprock for the Mt. Simon injection zone in the planned CO2 injection well and therefore, borehole enlargement had to be minimized to provide zonal isolation and containment for injection. After drilling the CO2 injection well, a comparison was made between the predrill prediction and actual results. Borehole enlargement was experienced at depths predicted by the predrill model, but borehole enlargement severity was minimized with improved drilling practices. To determine the formation stability, an MEM was built using offset well data and was calibrated with both borehole and regional data. Multiple stress models were examined, and calibration for unconfined compressive strength (UCS) values was provided from core tests in other wells. After calibration, the data was correlated to the planned well trajectory using formation tops, and the stresses were recalculated. The largest areas of uncertainty in the predrill prediction existed in the Eau Claire shale UCS and the maximum horizontal stress magnitude. Available shale core testing for UCS was likely biased toward stronger intact samples. The principle stress ordering and magnitudes from offset well testing showed a shallow thrust fault environment with deeper strike-slip environment.

Increasing mud weight to reasonable levels would not prevent shear failure in the Eau Claire shale as predicted by the models. Therefore, it was important to monitor the wellbore while drilling through the shale and mitigate borehole enlargement. For monitoring borehole enlargement, material across the shakers from the drilling process was examined for cavings. Digital pictures of the rock cuttings/cavings samples were taken before reaching the Eau Claire shale and through the interval. Mechanical vibration and backreaming were kept to a minimum across the Eau Claire shale while this section was open to minimize borehole enlargement.

Introduction

Geological sequestration of CO2 is being tested on several projects as part of a collaborative effort between the U.S. Department of Energy and private backers. In one project, the target injection formation is the Mt. Simon Sandstone, a saline water-bearing formation at 6,500 ft below the surface. The predrilling estimate of Mt. Simon's thickness was 1,200 to 2,000 ft with the upper part of the formation expected to have an average of 11% porosity and a 20-mD permeability, with the lower arkose zone possibly having a permeability over 1 D. In the proposed injection well, the Eau Claire shale just above the Mt. Simon sandstone provides the geological seal for injected CO2.

Many offset wells in the area were known to have borehole enlargement problems in Eau Claire shale. As part of the predrill planning for the injection well, an analysis was made to determine the safe mud-weight window that would prevent borehole failure. Vertical containment will be monitored in the St. Peter Sandstone just above the Eau Claire shale.

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