HOW TO PREDICT PORE PRESSURES AND FRACTURE GRADIENTS

Accurate prediction of pore pressures and fracture gradients has become almost essential to drilling deep wells with higher-than-normal pore pressures. Cost and drilling problems can be reduced substantially by recognizing abnormally high pore pressures early. The accurate determination of fracture gradients defines the need for protective casing and limits.

There are several methods to predict changes in pore pressure which is based on formation compaction or the lack of it. Compaction occurs primarily in shales; thus most prediction methods are based on what happens when drilling or logging shale sections. Normally the density of shales goes up as depth increases; If not, you will suspect abnormally high pore pressure.

Some primary methods used to detect pore pressure changes follow:

Seismic Data:

This technique relies more on normal formation compaction with depth. When this normal compaction trend is not followed, the velocity of sound waves is reduced. These velocity fluctuations can be detected and converted to the degree of adnormal pore pressures.

To be accurate, precise sound velocity data are needed. These may determine the successful application of this method. In some areas, seismic data have given erroneous results. Actually, seismic data have more utility in exploratory areas. Specific interpretations of seismic data are tough, and experienced geophysicists are necessary for meaningful outcomes. Any method of this type improves as field data are gathered. Therefore, drilling personnel and geophysicists can profit by working closely with one another.

Drilling Rate:

If  we can understand what the well is saying and react accordinly, many of the serious problmes encountered in drilling can be avoided. The drilling practices program needs to be fully undestood and the outcomes recorded accurately. As I said before, the well talks primarily through changes in monitoring devices, which include:

  • Pumping pressure and strokes as well as drilling rate recorders.
  • torque gauges and weight indicators.
  • pit level indicators and  mud properties.

When there is accumulation of rocks in the bit, we must solve the problem, By the time we recognize this, it may be tto late to use the obvious solution. For that reason the use of hydraulics program is played. For years, cleaning below the bit was considered mainly for economic reasons. Nowadays, it may be the key to survival in deep, high pressure wells. Changes in pore pressure are also related to changes in drilling rate.

Thus, the wat to better understand the well is to determine whether a change in drilling rate signals a variance in lithology ot a change in pore pressure. For this reason exploration personnel must know if lithology or pore pressure has changed, then cuttings should be circulated to the surface before drilling continues.REcognizing changes in pore pressure while drilling is important so mud weight can be altered before problems develop. Decreases in pore pressure may signal lower fracture gradients and a high vulnerability to underground blowout. Increases in pore pressure, if not contained by quick increases in mud weight, may result in difficult-to-control well kicks and subsequent blowouts.

A decrease in pore pressure may be signaled by a reduction in drilling rate. An increase in pore pressure may be indicated by a rise in drilling rate. Continuing with this simple approach, these changes in drilling rate may never occur if cleaning below the bit is inadequate. Drilling rate is affected by changes in lithologiy and drilling practices. Lithology changes may not be inmediately evident, and it may be necessary to stop drilling  and circulate cuttings to the surface. Recognizing lithology changes may depend on experience and also on abrupt changes in drilling rate.

The sudden increase in drilling rate shown in this graphic generally indicates a change in lithology, while the more gradual change in drilling rate generally is indicative of an increase in pore pressure.

To minimize mud underbalance in the pressure transition zone, drilling rate should be controlled and mud weight increased to keep the drilling rate below a normal drilling rate. Assuming good bottom-hole cleaning, drilling rate can be reduced by decreasing bity weight or rotary speed to reduce drilling rate immediately and then increase mud weight. This procedure makes it more difficult to accurately determine the required mud weight increase.

The drilling rate in the normal pressure region is extrapolated into the abnormal pressure region. With no change in bit weight or rotary speed, the mud weight increase required to reduce the drilling rate to the normal line should equal the pore pressure increase. A simply way to determine the pore pressure is simply to keep the drilling rate on the normal line by going  mud weight up. In most cases the problem is compounded because of reductions in bit weight or rotary speed.

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