Abstract
During hydraulic fracturing or acid fracturing treatments, the efficacy and performance of the stimulation fluid largely depends on the bottomhole injection temperatures to which the fluid is exposed as it enters the fracture. This study focuses on calculation of injection temperatures from available measurements to aid in the fluid selection and design for well stimulation operations.
Injection temperatures at the perforations during well stimulation treatment, is one of the starting points in fluid selection processes, be it acid or propped fracturing stimulation. In acid fracturing, inaccurate estimation can lead to incorrect selection of acid which can result in either an under-stimulated well, or in worst case, damage to the tubular goods resulting in expensive workover operations. In case of propped fracturing treatments, inaccurate estimation of wellbore temperatures can result in (a) an early or delayed cross-linking of fluid which can lead to shear-degradation of fluid or even a premature screen out, (b) higher than expected treatment pressures if cross-linking is delayed excessively leading to increased horsepower requirements and, (c) screen out or inferior stimulation due to overly conservative treatment designs. Accurate temperature estimates are thus a necessity.
The temperature distribution and prediction of temperature profile in the wellbore during injection process has been well studied and documented (Ramey, 1962; Wu and Pruess, 1990; Hagoort, 2004; Hassan et al. 2005) with original contributions by Carslaw and Jager (1959) on transient heat conduction in an infinite radial system. The current study uses temperature measurements, recorded by gauges that are typically located far away from the perforations, as inputs for the calibration of overall system heat transfer coefficient. Subsequently, based on well construction details and injection schedule, temperatures at perforations are predicted. The calculation can be carried out in a live mode if the treatment data is available in real time.
The paper discusses the pertinent theory and development of a calculation engine that can predict temperatures in the wellbore after calibration of the input data. Though predictions can be made for any depth in the wellbore, the focus is on the temperatures that exist at the perforation depths. The calculated injection temperatures were found to be different than measured values at shallower depths indicating that gauge temperature does not accurately represent injection temperatures at the perforations. Correct estimation is thus critical to the success of the treatment so that fluid properties can be adjusted and tailored to meet the requirements, and even possibly assist in optimizing the stimulation program.
Prediction of bottomhole temperatures at the injection point obtained with the help of temperatures measured at shallower depths is a unique approach and produces more accurate results than mere predictions based on generic assumptions. The newly developed approach can be applied to any stimulation treatment where temperature measurements are available.
