Squeeze cementing has been a common oil field operation for decades. During this time, technical improvements in squeeze job performance have been achieved through (1) a better understanding of hydraulic fracturing principles, and (2) utilization of fluid loss additives. In spite of these improvements, however, the common practice of interpreting downhole performance from surface pressure indications has remained arbitrary and imprecise. Typically, attainment of some specified final surface pressure is regarded as an indication that the well has been squeezed, with little or no regard for fracture gradient, frictional pressure, or hydrostatic pressure---all of which contribute to the actual downhole squeeze pressure.
Computerized simulation of squeeze cementing operations, which allows operators to take into account various well parameters, has recently been developed. Based on fluid densities and rheologies, fracture gradients, and well configuration, a prediction of non-squeeze surface pressure is provided for the entire job. This allows operators to obtain an accurate determination of when they are actually getting a squeeze--without excessively exceeding the fracture gradient--by noting when actual surface pressure rises significantly above the predicted pressure.
The simulator has proven particularly useful in squeeze job applications utilizing foam cement. A description of operation of the simulator is provided, including case histories comparing actual and predicted wellhead pressures.
Recent technical developments have helped provide a better understanding and control of what is actually occurring during the job during squeeze cementing of low fracture gradient formations. At the forefront of these developments are, two primary innovations: (1) foamed cement, a material which has been in use for several years, and (2) a computerized squeeze job design simulator, a more recent development in cementing technology.
Wells with formations so weak that they can not support the hydrostatic pressure of a full column of water can be squeeze cemented using ultra-low density foam cement. Slurries with densities as low as 4 lb/gal are formulated by incorporating an inert gaseous phase, typically nitrogen, which is homogeneously dispersed throughout the slurry. The computer simulator provides valuable assistance in planning and executing the squeeze job by allowing critical design parameters to be determined prior to actually performing the job. With a plot of predicted wellhead pressure in hand prior to the job, it is possible to more efficiently determine when a formation has been squeezed during the job. This is accomplished by comparing actual wellhead pressures to those predicted by the simulator.
When the actual pressure rises by 200 to 500 psi above the predicted value, it can be assumed that the well is being squeezed. Using this method, it is possible to detect an early squeeze and minimize the amount of cement used to obtain an effective squeeze. It also provides a means of determining that a squeeze has occurred at some pressure level, at 1,000 psi for instance, in cases where an arbitrarily predetermined higher value, 2.000 psi for example, would have been used to terminate the job. This improvement in accuracy can eliminate unnecessary multiple-squeeze jobs that are often required on a well because some predetermined wellhead pressure has not been obtained during the squeeze job.
