Sunday, November 19, 2017

Progress in Fracture Treatment Design chapter 1

The idea of hydraulicaly creating cracks in a pay zone to enhance production was developed in the 1920s by R.F Farris of Stanolind Oil and Gas Corp. He evolved the concept following a study of pressures encountered during squeezing of cement, oil and water into formations. In 1947, Stanolind (later Amoco Production Co.) performed the first experimental hydraulic fracture in the Klepper #1 gas well in Grant County, Kansas, USA. Deliverability of the well did not improve appreciably, but technique showed promiese, and the following year Stanolind presented a paper on the "Hydrafrac" process. Halliburton Oil Well Cementing Company obtained a license to the process and , in 1949 , performed the first commerical fracturing treatments, raising production of two wells "outstandingly". 

The method took off. By 1955, treatments reached 3000 wells per month, and by 1968, more than a half-million jobs had been performed. Today, hydraulic fracturing is used in 35 to 40% of wells, and in the United States, where the procedure is most widespread, it has increased oil reserves by 25 to 30%. Interest in hydraulic fracturing shows no signs of abating. Application of the technology is expanding from mainly low-permeability reservoirs to medium-to-high-permeability setting.

Hydraulic fracturing is the pumping of fluids at rates and pressures sufficient to break the rock, ideally forming a fracture with two wings of equal length on both sides of the borehole. If pumping were stopped after the fracture was created, the fluids would gradually leak off into the formation. Pressure inside the fracture would fall and the fracture would close, generating no additional conductivity. To preserve a fracture once it has been opened, either acid is used to etch the faces of the fracture and prevent them from fitting closely together, or the fracture is packed with proppant (usually sand) to hold it open. This article concentrates on the latter technique.

Today, a typical fracturing treatment uses thickned fluids pumped in stages. The first stage is a "pad" of water, a polymer and additives. Then comes the slurry, which is pad plus proppant-generally sand - in suspension. Different concentrations of proppant and volumes of slurry are pumped as the job progresses.  

Pressure exerted by the pad initiates and propagates the fracture. The slurry helps extend the fracture and transports poppant. The fracture gradually fills until the proppant packs into the fracture tip. At this point, the fracture treatment is finished and pumping stops. As pressure within the fracture declines, the fracture closes on the proppant pack, ensuring that it remains in place, providing a conduit for hydrocarbons. Productivity would be inhibited by viscous fluid in the pad and slurry that remains in the formation. However, when the fluid's high viscosity is no longer needed, the high temperature of the formation or special oxidizers cause the fluid "break" to a lower viscosity, allowing it to be produced back.

Hydraulic fracturing lies at the interface of fluid mechanics and rock mechanics. In the 45 years since the first fracture job, fluid science has advanced significantly. Treatment fluids have been diversified to handle many temperature, chemical and permeability conditions . Additives control a range of fluid properties, such as viscosity, pH, stability and loss of the fluid to the formation, called leakoff. Many proppants have been developed, from the standard silica sand to high-strenght proppants, like sintered bauxite and zirconium oxide particles, used where fracture closure stress would crush sand. 

Until recently, advances in rock mechanics lagged somewhat behind those in fluid technology. In the 1950s, there was no need for a rigorous theory of fracture propagation, the backbone of fracture treatment design. Low-volume, low-rate and low proppant concentration fracture simulation succeeded without careful design.But as treatments grew in size and complexity, operators needed more control. Today, more than ever, the expense of hydraulic fracturing requires that the operator knows how the formation will respond  to treatment, and whether the treatment design -the selection of pump rates, fluid properties, pumping schedule and fracture propagation model- will created the intended fracture. 

---to be continued 

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