Overview of Current Hydraulic Fracturing Design and Treatment Technology-Part 2

Abstract

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Introduction Hydraulic fracturing has played a major role in enhancing petroleum reserves and daily production. Fig. 1 portrays a simplified version of the "typical"fracturing process. It consists of blending special chemicals to make the appropriate fracturing fluid, then mixing it with a propping agent (usuallys and) and pumping it into the pay zone at sufficiently high rates and pressures that the fluid hydraulically wedges and extends a fracture. At the same time, the fluids carry the proppant deeply into the fracture. When done successfully, the propped open fracture creates a "superhighway" for oil and/or gas to flow easily from the extremities of the formation into the well. Note that the fracture has two wings extending in opposite directions from the well and that it is oriented more or less in the vertical plane. Other types (e.g., horizontal fractures) are known to exist. Some have been observed at relatively shallow depths [less than 2,000 ft (600 m)], but they comprise a relatively low percentage of the situations experienced to date. Hence, the discussion is directed primarily to vertical fractures. Fracturing technology requirements are multifaceted and are becoming more complex as our target formations get deeper, hotter, and lower in permeability. Fracturing state of the art over the past permeability. Fracturing state of the art over the past decade has changed continually to address the technological challenges that emerged with the development of massive hydraulic fracturing (MHF). This discussion covers much of the currently developing technology and the future needs for technological advances. Fracturing Fluids A fracturing fluid is used basically to:wedge open and extend a fracture hydraulically, andtransport and distribute the proppant along the fracture. The fluid(s) selected for a treatment can have a significant influence on the resulting effectively propped fracture length and fracture conductivity, as propped fracture length and fracture conductivity, as well a son the treatment cost. Fluid properties strongly govern fracture-propagation behavior and the distribution and placement of the propping agents. Fluids that leak off rapidly into the formation have a low efficiency in hydraulically wedging and extending a fracture. Fluid leak off also may leave an undesirable concentration of particulate residue in the fracture that could reduce fracture conductivity. The effective viscosity of the fluid controls the internal fracturing pressure and the propping agent transporting pressure and the propping agent transporting characteristics. Some desirable features of a fluid for most fracturing treatments include:low fluid loss to obtain the desired penetration with minimum fluid volumes;sufficient effectiveviscosity to create the necessary fracture width, and to transport and distribute the proppant in the fracture as required;no excessive friction in the fracture;good temperature stability for the particular formation being treated;good shear stability;minimal damaging effects on formation permeability;minimal plugging effects on fracture conductivity; plugging effects on fracture conductivity;low-friction-loss behavior in the pipe;good post-treatment breaking characteristics;good post-treatment cleanup and flow back behavior; and post-treatment cleanup and flow back behavior; andlow cost. JPT P. 853