Interesting Facts Regarding Well Depth, ECD During Drilling, Frac Gradient and Overpressure:
At a depth of more than 2000m the overpressure starts (departure from the hydrostatic pressure straight line gradient), and pore pressure (formation fluid pressure) increases rapidly with depth- similar to a balloon swelling effect. In an over-pressured reservoir, the porosity will be larger than normal, therefore the permeability will be larger, and formation (pore) compressibility will be larger. The deeper the formation is, the greater the tendency for the deeper formations to be hydraulically isolated from the shallower ones.By about 3800m TVD (12,467 ft TVD), pore pressure reaches a value close to the overburden stress, a condition referred to as hard overpressure. This implies that at depths beyond about 3000m TVD there will be a much smaller difference between the formation pore pressure and the pressure required to initiate a hydraulic fracture. It is therefore imperative that close attention is paid to mud circulation pressure during drilling to avoid unnecessary mud loss, and that is why accurate determination of Equivalent Circulating Density (ECD) is so important because the tolerance or margin difference between the pore pressure and fracture initiation pressure will be much less. Referring to an equivalent circulating “density” is a convenient way of addressing the additional pressure losses during mud circulation. Kiwigeo referred to fractures opening during circulation leading to apparent mud losses, but when the mud pumps are switched-off the fractures should close leading to an influx of mud and probably an increase in mud gas.
The formula for ECD is as follows:
ECD = ppg + dP/(0.052*TVD)
The formula for ECD includes a term to represent the static mud weight in pounds per gallon (ppg) and this term is often referred to as the ESD (Equivalent Static Density).The higher the barite content the higher the ESD in ppg.
The second term takes into consideration the forward circulation of mud up the annulus, which takes into consideration the annulus pressure drop dP, and the constant 0.052 converts ppg.ft into psia.
The greater the pump pressure and hence pump rate the greater will be the value of ECD.Other factors that can increase dP in the annulus include the length of open hole annulus between the formation wall and the drill pipe outer diameter and the tighter the annular clearance the greater this friction pressure loss will be.In addition, hole instability will likely result in an increase in dP and cause the ECD to be higher than intended.
