Pore and Fracture Data to Well Planning Engineer

Importance of Pore and Fracture Data to Well Planning Engineer

Here we will start by looking at different types of pressures and how they are used in drilling field. We will focus mainly on different types of pressures like formation /pore pressure, fracture pressure, hydrostatic pressure, fluid pressure and bottom-hole pressure. We will try to study and understand the circumstances leading to their arise (Hottman & Johnson, 1965 p720).

What causes Formation/Pore pressure

Formation pressure is the pressure of the liquid found in pore space of the formation segmented rocks.  The pressure is sometime affected by the kilograms of the rock layers on of formation rock that puts pressure on both formation liquids and solids. Pore pressure is known in the drilling of wells to be safe and affordable. But beside that it plays other major roles that enable it to reliable. These include: The movement of formation fluids and integrity seal as stated by (Mallick, 1999) before drilling related details of pore pressure is obtained from seismic information by first getting seismic speed and then utilizing speed to effective stress changing appropriate in a certain area mixed with estimated heavy stress to produce formation pressure.

If pore liquids escapes then the rocks looses its normal support and this forces them to contact together (compaction). One can describe it to be either normal or abnormal pressure depending on the quantity of the pore pressure produced. Normal pressure is equivalent to the hydrostatic pressure that is elongated from the surface to the considered surface. While abnormal pressure is pore pressure bigger than the hydrostatic pressure of formation that is contained in the pore spaces. Fracture pressure usually is the amount of pressure it takes to deform the grain structure of formation.  Some major disasters have resulted to loss of life and destruction of many important properties; this however has been controlled by pore pressure which has predicted likely disasters for example excessive pressure substance areas by using seismic studies. Seismic studies predict changes interval speeds with depth from speed analysis (Stork, 1992).

When burying, pressure sustain hydraulic that communicates to the underground. As a result sonic speed, resistivity and density of the actual pressure will increase with the deepness of burial and how the various properties of such kind’s rocks differ (Pennebaker.1968) In addition (Swan, 2001 p 1742) states that seismic measurements decide the velocity data of the variation reflection time at offset. Effective pressure which normally takes place on solid rocks is in charge of compaction of granular media which comprise of sedimentary types of rocks.

Causes of overpressure

Overpressures in sedimentary underground rocks are caused by varicose reasons, which results to increase in pressure and liquids that are generated. Therefore overpressure is generated according to the fluid and rock properties of sedimentary rocks in accordance with change rate under the usual basin reasons (Osborne & Swarbrick, 1997 p1032)

What increases stress?

When sedimentary rocks are deposited, vertical stress increases thus the pore liquids escape its own space trying to be combined. If it’s interfered with a layer of low water concentration for example clay which prevents the liquid to constant with the increase in vertical stress thus forcing the fluid to carry bigger components as a result increasing the pore pressure. This method is called under-compaction as stated by (Ratcliff&Roberts,2003) It is used in determining overpressure mechanisms by giving more detailed information on overpressure in high basins where frequent deposition and subsidence take place for example in Niger Delta and Mississippi areas.

Determining overpressure

Common used and reliable means of determining overpressure during construction of trend is called well logs method. Normally there is increase in density, increase in resistivity and increase in sonic speed after separating from the normal trends, or remain constant. This however takes place after the well has been drilled but usually it is important to detect overpressure before drilling takes place. Despite of being a precaution method it assists one to plan well and accordingly before starting to drill (Yilmaz, 2001). Seismic reflection is used because overpressure intervals have low speeds than the required pressure at same depth. Hence changes in pore pressure can noted by evaluation tools for example resistivity, sonic, density and porosity logs. The detailed logs show pore pressure effects due to the relationship between density, compaction, porosity and electrical properties of sedimentary rocks

Conclusion

Pore or formation pressure is applied in drilling oil and gas wells in the field. During mining they assist one to know the hydrocarbon traps and showing the maps of hydrocarbon roots trap to be configured and lastly assist in basin geometry and modeling. Overpressure in sedimentary rocks increase pressure and fluid that are generated. Well logs is the commonly and reliable method used in detecting overpressure in sedimentary rocks.

Notes

  1. Pore pressure is the same as pressure acting on fluid or formation pressure.
  2. Hence its equivalent to the hydrostatic pressure of water formation going underground.
  3. Gradient hydrostatic pressure increases when dissolved solvents are added to pore water.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Work Cited

Hottman, C.E. and Johnson, R.K., 1965, Estimation of formation pressures from log

derived shale properties, J. Pet. Tech. 6, June p717 – 722.

Mallick, S., 1999, some practical aspects of prestack waveform inversion using a genetic

algorithm: An example from east Texas Woodbine gas sand, Geophysics, V64, no.2,

P326-336.

Osborne, M.J. and Swarbrick, R.E., 1997, Mechanisms for generating overpressure in

sedimentary basins: a re-evaluation, AAPG Bulletin, V81,p1023-1041

Ratcliff, A. and Roberts, G., 2003, Robust automatic continuous velocity analysis, 73rd

Int’l Ann. Meeting, Soc. Expl. Geophys.. Dallas, p181-184

Pennebaker, E.S., 1968, Seismic data indicate depth and magnitude of abnormal pressure,

World Oil, 166, p73-82.

Swan, H.W., 2001, Velocities from amplitude variations with offset, Geophysics, 66, no.6,

P1735-1743.

Stork,C., 1992, Reflection tomography in the post migrated domain, Geophysics, V57, no.

5, p680-692.

Yilmaz, O., 2001, Seismic data processing, Soc. Expl. Geophys. Publication

 


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