Energy-based formation pressure prediction

Abstract

Conventionally, pore pressure predictions from the drilling parameters have the advantage of estimating the formation pressure at the bit at relatively low cost. The limitations on the application of d-exponent concept to pore pressure prediction have long been established. Recent development in pore pressure prediction from the drilling parameters uses the concept of mechanical specific energy (MSE) and hydro-rotary specific energy (HRSE). These energies are usually computed from the downhole measurements. However, majority of readily available field data in older (offset) and present-day wells are in the form of surface measurements. In this paper, a new pore pressure prediction technique based on the concept of hydro-mechanical specific energy (HMSE) is being proposed. The HMSE is the combination of axial, rotary and hydraulic energies required to break and remove a unit volume of rock. The new technique uses drilling parameters that are obtained only from surface measurements. Pore pressure prediction using the concept of HMSE is based on the theory that total energy consumed in breaking and removing a unit volume of rock beneath the bit is a function of effective stress: the higher the effective stress, the greater the total energy required to break and remove a unit volume of rocks. Abnormally high formation pressure intervals with lower effective stress will require less energy to drill than the normally compacted series at the same depth. The new technique is test using a recently drilled near vertical deep High-Pressure High-Temperature (HPHT) exploratory well in the Tertiary Deltaic System of the Niger Delta basin where the main cause of overpressure mechanism is under-compaction. The well drilled to a total depth of more than 17,000 ft-TVD covers both the normally pressured and overpressure intervals. Pore pressure estimates derived from the HMSE concept are then compared to the actual pore pressure measurements taken from the formations of interest. There is an excellent agreement between the predicted and measured formation pore pressure. The new technique can provide a reliable means of estimating the formation pore pressure from the drilling parameters in absence of reliable downhole measurements at relatively low cost.