Pore pressure

An important safety feature on every modern rig is the blowout preventer (BOP). As discussed earlier on, one of the purposes of the drilling mud is to provide a hydrostatic head of fluid to counterbalance the pore pressure of fluids in permeable formations. However, for a variety of reasons (see section 3.6 Drilling Problems ) the well may kick , i.e. formation fluids may enter the wellbore, upsetting the balance of the system, pushing mud out of the hole, and exposing the upper part of the hole and equipment to the higher pressures of the deep subsurface. If left uncontrolled, this can lead to a blowout, a situation where formation fluids flow to the surface in an uncontrolled manner. 

Filtration installations include wrapping the trench of a pavement-edge drain system to prevent contamination of the underdrain placement behind retaining walls and bridge abutments to prevent contamination of the sand blanket placed against the stmcture to allow dissipation of pore pressures in order to avoid failure of the stmcture as silt fences to allow surface mnoff from a site while retaining the soil suspended in the mnoff and on earth slopes beneath larger stone or other overlay materials to prevent erosion of the slope as water escapes from the interior of the slope. 

Subsurface Fluid Pressure (Pore Pressure Gradient). The total overburden pressure is derived from the weight of the materials and fluids that lie above any particular depth level in the earth. Of interest to the petroleum industry are the sedimentary rocks derived from deposits in water, particularly, in seawater. Such sedimentary rocks contain rock particle grains and saline water within the pore spaces. Total theoretical maximum overburden pressure, P (Ib/ft-), is... 

The fluid pressure in the rock at the bottom of a well is commonly defined as pore pressure (also called formation pressure, or reservoir pressure). Depending on the maturity of the sedimentary basin, the pore pressure will reflect geologic column overburden that may include a portion of the rock particle weight (i.e., immature basins), or a simple hydrostatic column of fluid (i.e., mature basins). The pore pressure and therefore its gradient can be obtained from well log data as wells are drilled. These pore pressure data are fundamental for the solution of engineering problems in drilling, well completions, production, and reservoir engineering. 

Because the geologic column of sedimentary rock is usually filled with saline water, the pore pressure and pore pressure gradient can be obtained for nearly the entire column. Figure 2-57 shows a typical pore pressure gradient versus depth plot for a Gulf Coast region well. 

Subsurface Rock Fracture Pressure (Fracture Pressure Gradient). The subsurface rock fracture pressure can be approximated by utilizing the known pore pressure at the same depth. The relationship between rock fracture pressure p (psi) and pore pressure p (psi) is [34]... 

In Figure 2-57 the pore pressure gradient has been given as a function of depth for a typical Gulf Coast well. Determine the approximate fracture pressure gradient for a depth of 10,000 ft. From Figure 2-57, the pore pressure gradient at 10,000 ft is... 

In general. Equation 2-174 can be used to approximate fracture pressure gradients. To obtain an adequate approximation for fracture pressure gradients, the pore pressure gradient must be determined from well log data. ALso, the overburden stress gradient and Poisson s ratio versus depth must be known for the region. 

Control mud weight to counter balance pore pressure. Keep fluid loss as low as possible. Keep viscosity and gel strength low to prevent swabbing. 

No loss-of-circulation problem No formation damage Very high penetration rate Low bit costs Low water requirement No mud requirement No ability to counter subsurface pore pressure problems Little ability to carry formation water from hole Hole erosion problems are possible if formations are soft Possible drill string erosion problems Downhole fires are possible if hydrocarbons are encountered (gas only) Specialized equipment necessary... 

Improved penetration rates (relative to mud drilling) Ability to counter high subsurface pore pressure problems ... 

In most air and gas drilling operations, open-hole well completions are common. This type of completion is consistent with low pore pressure and the desire to avoid formation damage. It is often used for gas wells where nitrogen foam fracturing stimulation is necessary to provide production. In oil wells drilled with natural gas as the drilling fluid, the well is often an open hole completed with a screen set on a liner hanger to control sand influx to the well. 

Air and Gas. In the regions where air and natural gas are used as the principal drilling fluids, the potential oil and gas production zones usually have low pore pressure, or require well stimulation techniques to yield commercial production. In these production zones, air drilling (or natural gas drilling) is continued into the production zone and the initial produced formation fluids are carried to the surface by the circulating air or natural gas. This is nearly the same situation as in mud drilling, except that in air (or gas) drilling the transit time for the initial produced formation fluids to reach the surface is much shorter. In mud... 

Stable Foam. When a well is drilled with stable foam as the drilling fluid, there is a back pressure valve at the blooey line. The back pressure valve allows for a continuous column of foam in the annulus while drilling operations are under way. Thus, while drilling, this foam column can have significant bottom-hole pressure. This bottomhole pressure can be sufficient to counter formation pore pressure and thus control potential production fluid flow into the well annulus. 

Gqb overburden gradient in psi/ft Gp = pore pressure gradient in psi/ft K = coefficient related to Poisson ratio... 

The d, exponent is mostly used in Eaton s equation for a quantitative estimation of the pore pressure using the following equation [122] ... 

Other attempts to develop a pore pressure evaluation method from different drilling equations were made by Combs, by Bourgoyne and Young and by Bellotti and Giacca [101]. The three models follow the same general approach a drilling equation is developed assuming that all variables are independent. The ROP is then normalized to eliminate the effect of each variable but the pore pressure. These equations attempt to take more variables into account. 

Hottman and Johnson developed an empirical correlation to relate the ratio of resisitivities to the pore pressure gradient. In 1972, Eaton developed an empirical relationship that he modified in 1975 to the following [122] ... 

Figure 4-330. Matthews and Kelly relationship between formation pore pressure and shale resistivity for the South Texas Gulf Coast. (Courtesy SPE [101]. ...

Figure 4>332. Pore pressure evaluation in a North Sea weil. (Courtesy Louisiana State University [124].)...

Compute the pore pressure at 15,000 ft using Eaton s equation, a normal gradient of 0.453 psi/ft, and an overburden gradient of 1 psi/ft assuming the well vertical. 

Drilling break. A relatively sudden increase in the instantaneous drilling rate is called the drilling break. The drilling break may occur due to a decrease in the difference between the borehole pressure and formation pore pressure. When a drilling break is observed, the pumps should be stopped and the well watched for flow at the mud line. If the well does not flow, it means that the overbalance is not lost or simply that a softer formation has been encountered. 

Since the hydrostatic pressure of the original mud is 5,214.7 psia, the reduction in the hydrostatic pressure is about 69 psi. Because the pore pressure at the vertical depth of 10,000 ft is 4,700 psi, the hydrostatic pressure of the gas-cut mud is sufficient to prevent any formation fluid kick into the hole. 

Currently, a graphical method of casing setting depth determination is used. The method is based on the principle according to which the borehole pressure should always be greater than pore pressure and less than fracture pressure. 

It is suggested to evaluate the burst load based on the internal pressure expected, reduced by the external pressure of the drilling fluid outside the string. Internal pressure is based on the expected bottomhole pressure of the next string with the hole being evacuated from drilling fluid up to a minimum of 50%. In exploratory wells, a reasonable assumption of expected formation pore pressure gradient is required. 

Alixant, J-L., Real-time Effective Stress Evaluation in Shale Pore Pressure and Permeability Estimation, Ph.D. dissertation, Louisiana State University, p. 210, December 1989. 

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