Development and Testing of Spent Shale Grouts for Backfilling Abandoned In-Situ Oil Shale Retorts

Abstract

Summary Grouts based on Lurgi spent shale were developed for backfilling in-situ oil shale retorts. Lignosulfonate fluidizer (0.25%) was used to achieve adequate fluidity. Addition of 10% fly ash increased the strength of the grouts. Permeabilities of the grouts decreased at higher confining pressures and were low enough to reduce leaching by groundwater. Introduction Water-quality problems of oil shale retorting are mainly related to the ultimate disposal of spent shale. In in-situ retorting, spent shale remains underground, where it can be leached by reinvading groundwater. To minimize leaching, it has been proposed to backfill abandoned retorts with a grout, reducing their permeability. Mallon discusses a possible grout injection procedure. The large volume of voids to be filled requires that the grout be based on spent shale. A hydraulic cement has been produced by calcining a mixture of spent shale and limestone, but the requirements for importing limestone to the site make this uneconomical. Therefore grouts were developed that contained only spent shale from a demonstrated retorting process and small amounts of additives. Materials. Low permeability of a noncementitious grout results not from chemical properties but from fine particle size. The finest spent shale is the fraction of spent particle size. The finest spent shale is the fraction of spent shale from the Lurgi process that is collected by electrostatic precipitation. This material was 57.7% finer than 0.00394 mm [0.00016 in.]. Analysis by X-ray diffraction showed that the major minerals present were quartz, calcite, dolomite, and feldspar (same as in raw shale). Free lime and silicates were not detected. A pozzolanic Class F fly ash and a pozzolanic, pozzolanic Class F fly ash and a pozzolanic, cementitious Class C fly ash with 5% added CaSO4 were tested as strength-increasing additives. Two lignosulfonate fluidizers (refined sodium salts of lignosulfonic acid, a waste product from the sulfite pulping process) were tested: Crown-Zellerbach product 512 pulping process) were tested: Crown-Zellerbach product 512 is desugared; Crown-Zellerbach product 503 has not been desugared (sugars are removed because they retard the setting of portl and cement). Grout Fluidity and Bleeding. Formulating grouts for injection into rubble is limited by two opposing constraints. Bleeding (separation of a clear supernatant) must be as low as possible, but the grout must still be fluid enough to penetrate through the voids of an abandoned in-situ retort. Particulate grouts are non-Newtonian fluids with nonzero yield stress, which limits penetration of grout through small pores. Bleeding penetration of grout through small pores. Bleeding increases and yield stress decreases with increasing water/solids ratio (WSR). Thus the objective is to formulate a grout that has a low enough yield stress without unacceptable bleeding. Yield stress was not used directly as a criterion. Rather, grouts were formulated to have flow cone times between 18 and 22 seconds. Experience with intrusion grouting of preplaced aggregate concrete has shown this to be a realistic requirement. Experiment. Grout ingredients (Table 1) were dry blended and water was added in increments to reduce the flow cone time to the target range. Grouts were mixed at 1,300 rev/min for 3 minutes initially and after each incremental addition of water. Bleeding was less than 2 % for all the grouts. Samples were cast in 2-in. ×4-in. [5. 1-cm× 10.2-cm] waxed cardboard cylinders for both permeability and strength testing. Permeability samples contained no aggregate because the permeability of a grouted retort would be controlled by the grout (the continuous phase) rather than the in-situ spent shale (the discontinuous phase). Simulated grouted cores for strength phase). Simulated grouted cores for strength measurement were prepared with grout and aggregate (simulated in-situ spent shale from Laramie Energy Technology Center retort run 5–55, crushed and sieved to - % + 1 in. [0.64 to 0.95 cm]). All samples were cured in 100% relative humidity for 5 months at 73 deg. F [23 deg. C]. Permeability was measured at a range of confining Permeability was measured at a range of confining pressures (Fig. 1) and electrical conductivity of pressures (Fig. 1) and electrical conductivity of permeates produced during these tests was measured. permeates produced during these tests was measured. Experimental details are presented in Ref. 13. Discussion. The grout formulas, flow cone times, and the unconfined compressive strengths are shown in Table 1. The permeabilities are shown in Fig. 1. Points shown in Fig. 1 are measurements on duplicate specimens of each grout at three different confining pressures (data points for R-4 overlap). r = product-moment correlation points for R-4 overlap). r = product-moment correlation coefficient. The conductivity of leachates produced during permeability measurement decreased over time as shown in Fig. 2. JPT P. 1033