Computerizes monitoring of hydraulic fracturing treatments has been an accepted practice for several years. Not only can a continuous record of the treatment parameters be made, but the real-time bottomhole treating pressure can be determined without the use of a reference string or bottomhole pressure tool. However, to calculate the bottomhole treating pressure, the friction pressure of the fluid or slurry must be determines. For conventional, incompressible oil and water-based fluids friction pressure is a straightforward calculation since rate and proppant concentration are the main considerations. The friction pressure for foam fracturing fluids requires a more complex solution.
A computer van was used to monitor carbon dioxide and nitrogen foam treatments incorporating a reference string or bottomhole pressure tool. The following parameters involved in the calculation of bottomhole treating pressure were examined in this study: injection rate, addition of proppant, effect of proppant on slurry friction pressure and foam hydrostatic head pressure. This paper presents a technique which allows the bottomhole treating pressure to be calculated without the use of a reference string or bottomhole pressure tool. The calculation of bottomhole treating pressure for a foam treatment incorporates a technique to correct the hydrostatic head pressure of carbon dioxide and nitrogen foams. In addition, the different effects of proppant addition on friction pressure between nitrogen and carbon dioxide generated foams are discussed.
The knowledge and interpretation of bottomhole treating pressure has become an important factor in the fracture stimulation of oil and gas wells, since the initial work of Nolte and Smith was published in 1979. Qualitative analysis of fracture propagation by many authors - has been made using the real-time net bottomhole treating pressure. In addition, treatment design parameters such as the fluid loss coefficient, fracture height, length, width and closure time may be determined using the post-fracture pressure decline analysis. Veatch and Moschovidis report on the recent advances in fracture analysis and diagnostic technology using bottomhole treating pressure.
Initially, bottomhole treating pressures were measured with a reference string. A reference string is a tubing or annulus which is filled with a fluid or gas to determine bottomhole treating pressure. The necessity of running a reference string, the tubing with the treatment pumped down the tubing-casing annulus or the annulus with the treatment pumped down the tubing, adds considerable cost to the fracturing treatment. The addition of a tubing string may create enough flow restriction to cause the desired treatment to be operationally unacceptable. Wire line tools are available to measure the bottomhole treating pressure. The use of a tool eliminates the restriction created by the tubing string and usually reduces treatment costs.
The implementation of on-site computers has greatly facilitated the gathering of hydraulic fracturing-treatment data and subsequent analysis. The monitoring technique measures surface variables such as surface treating pressure, fluid injection rate and proppant concentration and then calculates a bottomhole treating pressure as described by Hannah and Harrington, et al.