Summary
As a type of mono-alkyl ester, biodiesel exhibits great potential to serve as the base oil of drilling fluids substituting for conventional oil-based drilling fluids (OBDFs). This paper presents a series of laboratory investigations of water-in-biodiesel (invert) emulsion as the basis of a high-performance, environmentally friendly, and low-cost biodiesel-based drilling fluid (BBDF). Biodiesel produced from waste cooking oil was used to formulate a BBDF because of its high flashpoint, reliable storage stability, acceptable elastomeric material compatibility, nontoxicity, and excellent biodegradability. In light of the results of tests used to measure various properties, the biodiesel invert-emulsion chemistry, including the required hydrophile/lipophile balance (HLB), optimal emulsifier, effects of different additives (organophilic clay, calcium chloride, and lime), as well as hydrolytic stability, was studied. A biodiesel invert emulsion that remains stable after hot rolling at 120°C for 16 hours can be prepared with correct combinations of additives, thereby offering a firm foundation for designing BBDFs. The novel emulsifier package developed in this work is introduced as an achievement in the comprehensive usage of waste cooking oil because its feedstock is identical to that of biodiesel. An initial economic analysis of the use of biodiesel for drilling is also presented. Details of the formulations and properties of BBDFs derived from this fundamental research are discussed in another paper (Part 2).
The
low permeability of coal seams is a key factor restricting
gas extraction. The multiscale pores in low-permeability coal make
coal permeability present the multiscale characteristics. However,
the conventional steady-state method cannot measure the multiscale
permeability of low-permeability coal well. In this study, a unidirectional
multiscale dynamic apparent diffusion model is proposed as an analytical
model, and a multiscale dynamic apparent diffusion coefficient is
defined. In addition, an experimental method for measuring low permeability
from macroscale to microscale pores is provided. The experiments of
gas desorption flow in the unidirectional, radial, and spherical directions
were conducted to compare with each other. The research results show
that (1) the apparent diffusion coefficient and apparent permeability
decrease with time because of the multiscale pore structure in coal.
(2) The multiscale dynamic apparent diffusion model can accurately
describe the full-time process of the unidirectional gas desorption
flow in coal. (3) The proposed model shows a broader applicability
with a comparison to the current models.
Permeability is the most important property that controls the transfer of gas mass across a hierarchy of scales within a shale gas reservoir. When gas diffuses from the fracture wall into the matrix, the gas adsorbs onto shale grains. This adsorption may result in matrix swelling. In previous studies, it is commonly assumed that this swelling is uniform within the matrix. Under this assumption, the impact of the gas diffusion process would be neglectable. In this study, we hypothesize that this uniform swelling assumption is responsible for the inconsistencies between poroelastic solutions and experimental or field observations as reported in the literature. We introduce a volumetric ratio of the gas-invaded volume to the whole matrix volume to quantify the impact of matrix swelling volume expansion on the evolution of shale permeability. The gradual matrix pressure increase in the vicinity of fracture walls leads to local swelling. As the gas invaded zone expands within the matrix, the local effect weakens. When the matrix is completely invaded by the injected gas, a new homogeneous state is achieved, and the local effect ends. We find that the evolution of shale permeability from initial to final homogeneous states is a result of the propagation of the gas invaded area. We apply this approach to generate a series of shale permeability maps. These maps explain experimental observations under a spectrum of conditions from constant confining pressure, to constant average pore pressure, to constant effective stress, and to constant total volume conditions.
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