Hydraulic pressure stimulation

Hydraulic pressure stimulation (fracturing) of oil and gas wells has now accumulated 40 years of history and experience. The actual practice and application of this technique supports a multi-billion dollar service industry which annually utilizes in excess of 130 million pounds of chemical additives. This chapter will describe the fracturing fluids that are used and some of the additives, their purpose, and the principles that make their use effective as well as necessary. Information presented will update a body of review literature that covers the prior years of fracturing(1). Chemicals are added for specific purposes which are identifiable by their descriptive title. Veatch02) has compiled a thorough general list of the additives added to fracturing fluids. 

Hydraulic fracturing is a method of stimulating production of oil or gas from rock formations. A fluid is pumped under conditions of high pressure and high rate Into the formation to fracture it. The fluid also carries sand or a similar proppant material into the fractures. When the pumping is stopped and the hydraulic pressure is released at the wellhead, the fracture partially closes on the sand leaving a highly permeable channel for the oil or gas to flow back to the well. 

Besides that, compressive force can be manipulated to facilitate cell lysis and collect their cellular components for later cell-based assay. Similarly, Wang et al. fabricated a micromechanical stimulator fliat is capable of providing controlled compressive or tensile strain to the cultured cells in vitro (Fig. 4). Experimental characterization of its PDMS membrane deformation showed that the microdevice can provide —6 % compressive to 25 % tensile radial strain to the cultured cells within the membrane center [6], which allowed simultaneous investigation of both mechanical strains on the same cells. Zhou et al. developed a microchip platform with microchannels that resembles the mechanical environment of small blood vessels in vivo (Fig. 5). They demonstrated that the deformation of the membrane by hydraulic pressure induced cyclic circumferential strains on the adhered mesenchymal stem cells and thus caused significant stmctural and biochemical changes to the cells [7]. 

Hydraulic fracture stimulations are overseen continuously by operators and service companies to evaluate and document the events of the treatment process. Every aspect of the fracture stimulation process is carefully monitored, from the well head and downhole pressures to pumping rates and density of the fracturing fluid slurry. The monitors also track the volumes of each additive and the water used and ensure that equipment is functioning... 

A Comprehensive understanding of the fracture distribution and hydro-geomechanical processes occurring during hydraulic stimulation is essential in both conventional and Hot Dry Rock (HDR)/ Hot Wet Rock (HWR) geothermal development. Some of this information can be obtained from well logs such as flow, temperature, pressure, BHTV, FMI, etc. However, they only provide restricted information near the well. 

Figure I. Schematic illustration of a high temperature-pressure autoclave system used for simulated laboratory hydraulic stimulation...

Examples of the borehole pressure records obtained from the simulated hydraulic stimulation tests are shown in Fig. 2. Figure 2(a) shows the variation of the borehole pressure with respect to time at a room temperature (25 C), under the confining pressure of lOOMPa and the injection rate of 5.0 mmVsec. Figure 3 (a) shows the fracture profile observed on the cross section perpendicular to the specimen axis. The specimen has been impregnated with a red dye penetrant for ten... 

Figure 2. Representative curves of borehole pressure versus time for simulated hydraulic stimulation tests conducted at (a) a room temperature (25°C) and (b) 600°C. The confining pressure is 100 MPa, and the injection rate is 5 mm3/sec.

The occurrence condition of shear fracture is examined on the basis of the Coulomb criterion. The averaged shear stress across the fracture plane in the simulated hydraulic stimulation tests is plotted in Fig. 5, as a function of the effective normal stress across the fracture plane. The steady-stale pore pressure distribution given from Equation (1) is averaged over the fracture plane and is used to compute the effective normal stress. Triaxial compression tests have been performed on the granite using the same apparatus shown in Fig. 

I (Takahashi, 2000). The peak shear stresses obtained from the triaxial compression tests are also plotted in Fig. A2. In the plot of Fig. 5, there is general agreement between the two types of the experimental results. Thus, it is thought that the occurrence of the shear fracture in the simulated hydraulic stimulation tests can be approximately predicted by the Coulomb criterion. Based on the comparison, the critical condition for the shear fracture due to hydraulic stimulation was estimated using the experimental results of the triaxial compression tests, as given in Fig. A2, and the averaged value of pore pressure. The detailed discussion of the triaxial compression tests can be found elsewhere (Takahashi, 2000.). 

Simulated hydraulic stimulation tests were performed using thick-walled cylindrical specimens of lidate Granite at temperatures up to 600°C and confining pressures up to 100 MPa. The objective of the laboratory experiment was to examine the feasibility of creating an artificial... 

In this paper, we suggest a method for estimation of pore-pressure during hydraulic stimulation of geothermal reservoir by using the induced microseismic multiplets. The results of the application to the microseismic events at Soultz field are demonstrated showing the feasibility of the method. 

We estimate distribution of critical pore-pressure of fractures for shear slip during hydraulic stimulation. 

Because of the length of exposed wellbore, it is usually not possible to maintain a downhole pressure sufficient to stimulate the entire length of a lateral in a single stimulation event (Overbey et al., 1988). Because of the lengths of the laterals, hydraulic fracture treatments of horizontal shale gas wells are usually performed by isolating smaller portions of the lateral. 

Hydraulic fracturing A well-stimulation method in which liquid under high pressure is pumped down a well to fracture the reservoir rock adjacent to the wellbore. Propping agents are used to keep the fractures open. 

---