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A number of cementitious materials used for cementing wells do not fall into any specific API or ASTM classification.These materials include: Pozzolanic materials include any natural or industrial siliceous or silico-aluminous material, which will combine with lime in the presence of water at ordinary temperatures to produce strength-developing insoluble compounds similar to those formed from hydration of Portland cement. Typically, pozzolanic material is categorized as natural or artificial, and can be either processed or unprocessed. The most common sources of natural pozzolanic materials are volcanic materials and diatomaceous earth (DE). Artificial pozzolanic materials are produced by partially calcining natural materials such as clays, shales, and certain siliceous rocks, or are more usually obtained as an industrial byproduct. Pozzolanic oilwell cements are typically used to produce lightweight slurries.
Abstract Negatively charged poly(methyl methacrylate-co-butyl acrylate) (P(MMA-BA)) particles, and positively charged dissolved poly(ethyleneimine) (PEI) polymer were adsorbed onto a cement block using a layer-by-layer assembly technique. The block was fashioned so as to have a cylindrical hole running from one face to another along its long axis, and a fluid containing one of the two charged materials was pumped through the block. The result was a controllable-thickness film tens of micrometers thick, and the pressure required to crack the cement block was measured after sealing one end of the hole. Latex particles with a Tg near the use temperature showed the maximum improvement in the cracking stress of the blocks. In a multilayer coating with identically sized particles, the cracking stress of the blocks increased to an improvement of 25% and then dropped off with increasing number of layers, even though the relationship between film thickness and the number of layers was linear. An improvement of about 30% in the cracking stress of the coated blocks was obtained when using multiple layers with different particle sizes. Tests done under confinement, e.g., with an external stress applied to the outside of the blocks, suggest that not only does a film-forming mechanism contribute to performance, but filling of microcracks in the rock may also play a role.
Blair, S.C. (Earth Sciences Department, Lawrence Livermore National Laboratory) | Thorpe, R.K. (Earth Sciences Department, Lawrence Livermore National Laboratory) | Heuze, F.E. (Earth Sciences Department, Lawrence Livermore National Laboratory) | Shaffer, R.J. (Earth Sciences Department, Lawrence Livermore National Laboratory)
We have performed a series of laboratory tests to study the propagation of a hydrofracture into and through an interface between two rock-like materials. Test specimens were prepared by embedding sandstone tablets (lenses) in blocks of gypsum cement. These blocks were hydrofractured under true triaxial loading conditions, at a constant fluid injection rate. The injection path and applied state of stress were designed so that we obtained a single-wing fracture, propagating in a plane perpendicular to the interface. Fracture growth was tracked via extension failure of fine tungsten wires embedded in the gypsum. After testing, the blocks were dissected and the extent of fracturing and fluid leakoff were recorded. In each test, the hydrofracture propagated into and through the discontinuity. Pressure-time and fracture tracking data were consistent for all tests. Distinct step increases on the pressure-time record were also noted in all tests, and are related to the interaction of the hydrofracture with the sandstone lens. All the fractures showed step-crack behavior upon entering or exiting the sandstone tablet.
In addition, a finite element model was used to simulate the experiments. Pressure-time records from the model and the experiments compare favorably.
Hydraulic fracturing is a widely used technique for obtaining enhanced production from oil and gas wells and for measuring stresses at depth. Nevertheless, a poorly understood aspect of hydraulic fracturing is the behavior of a fracture as it approaches and intersects a material interface. For applications such as recovery of natural gas from lenticular sand formations, it would be useful to recognize fracture propagation through a geologic interface from features of the pressure-time record. To that end, a joint experimental and numerical modeling program has been pursued at LLNL. The experimental work is performed in support of the numerical modeling, because without physical validation, numerical models do not have credibility. We have completed scaled hydrofracture tests in blocks containing sandstone lenses, and tracked the fractures in a triaxial applied stress state . In this paper we present the results of the physical experiments and the simulation of the tests with the numerical model.
Test specimens were prepared by embedding tablets of Berea sandstone in blocks of gypsum cement. These blocks measured 29.2 cm x 29.2 cm x 45.7 cm (11.5 in. x 11.5 in. x 18 in.), and were tested under true triaxial loading conditions. A schematic of the block tests is shown in Figure 1. Flatjacks were used to apply confining pressure to the faces of the block. Test blocks and flatjacks were assembled in the confining cell as shown in Figure 2. Fluid was injected into the blocks at a constant rate using a high pressure, ball-screw piston pump developed for this project. Through special design of the injection hole , we enabled a single wing planar fracture to propagate perpendicular to the interface. The fracture was contained in the vertical direction by wire mesh screens located near the top and bottom of the gypsum block. Growth of the fracture was tracked via breakage of fine tungsten wires embedded in the gypsum. Location of wires in the blocks is shown schematically in Figure 3. After fracturing, the blocks were dissected to show the exact fracture outline and the extent of leakoff.
To increase the safety and productivity of underground coal mines, the U.S. Bureau of Mines, through an in house research project beginning in lg75, demonstrated the feasibility of using fast setting hydraulic cements for grouting coal mine roof bolts. The Colorado School of Mines was funded through the Spokane Mining Research Center to select, test, and demonstrate inorganic cements for this application. A grout composed of gypsum cement and potassium sulfate accelerator was selected as having the best properties for roof bolting. This grout is rapid setting, and reaches the yield strength of roof bolts in about three minutes from the time of mixing the cement. This cement system was found through testing of a variety of cements in combinations with several types of accelerators through a series of laboratory tests. The laboratory tests consisted of penetration, shrinkage, cube strength and pull strength tests. Field tests were conducted in Sommerset Mine of the U.S. Steel Corporation. Two intersections were used to test and demonstrate the cement-grouted bolt system. One intersection was supported with only cement-grouted bolts and an adjacent intersection with resin roof bolts in the same bolting pattern. These two intersections were instrumented to measure roof parting. It was found that the roof sag was essentially the same with a magnitude of less than .6 inches for the two intersections. Field visual inspection of these intersections was also conducted and no apparent differences were found. On the basis of this successful field demonstration further testing and development of production installation methods is recommended. Also additional testing will be required before government approval can be expected.
This paper will establish the role of cement roof bolt grouting in coal mines by a brief discussion of rock bolting in general. Then the procedure and results of the cement grout project will be described. A more detailed compilation and analysis of the laboratory and field testing can be found in the final report to the U.S. Bureau of Mines, available soon from the National Technical Information Service.
The object of rock bolting is to bind together a discontinuous rock mass to increase the stability around an underground opening. Two types of action have been attributed to rock bolts:
This chapter describes the development of a rocklike model material for use in tests with a rock-blocks model to investigate the failure mechanism of a discontinuum. In order to provide reliable results, the model material must be characteristic of natural rock and obey the laws of similitude. Initially, the development of such a material does not appear to be a difficult problem. Only when one undertakes such a task does he realize how little is actually known, and that there is considerable work yet to be done.
A material that simulates rock in all of its physical properties probably will never be developed. There is now and probably always will be room for improvement. Improvement is needed not only in the component materials but also in the technique of preparation.
The material developed and subsequently described in this chapter simulates rock in general and a sehistose gneiss rock in particular. The material possesses many rocklike physical properties not found in previously used model materials. One of the most significant improvements of this material is that there is an increase of Young's modulus with an increase in confining pressure as determined by triaxial compression tests. A change in the usual procedure of material preparation was used to develop this material and is also described.
MODEL MATERIALS USED IN THE PAST
Numerous materials have been used by investigators in models in the past to simulate a wide range of prototype materials. Some of the materials used include cork, plastic, concrete, plaster of Paris, Portland cement mortar, pumice, rubber, gelatin, etc. Each was selected to simulate the prototype material under investigation. Some of the above materials have a ductile failure, others a brittle failure. Some of these materials along with various admixtures have been used to simulate rock.
The literature is not without papers describing the development of materials used to simulate rock. However. most of these materials have been developed to simulate only one or two rock properties, and are not rocklike in all respects. In many cases the material developed has been considered "adequate" for the problem at hand and has been used. The materials used most frequently are either portland cement or a gypsum cement with various filler materials.
Some investigators have been interested only in developing a material whose modulus of deformation scales satisfactorily to the prototype rock. These materials have been adequate since techniques have been developed for altering (reducing) the modulus of deformation by means of drilling a large number of holes in the model material mass. These materials have been used to study the deformation of rock abutments for dams. However, in most of these investigations one very important property or relationship has been overlooked or neglected--the behavior of the material under an increase in confining pressure. This behavior can be observed by comparing the shape of the stress-strain curves obtained at different confining pressures or by a plot of Young's modulus vs. confining pressure. Only one or two investigators have recognized this as a problem and mention it in the literature.