Pressure Engineering

Pressure engineering, the science of predicting and and interpreting pore pressure is often part of the mudloogin contract and to certain extent, subject to the Wellsite geologist’s supervision. The pressure engineer in the mud loggin unit usually reports to the geologist and the company man, because his findings have direct bearing on both the drilling and geological interpretation. Again, cooperation is appreciated.

If no pressure engineer in on location, as it may be the case on many development well, the geologist takes care reporting indications of abnormal pressure the campany man,

During Drilling.

Little is known about the actual formation pore pressure. Unless a RFT or DST yielding directly measured formation pressures, has been conducted, all information about formation pore pressure is inferred form empirical formulas. Drill ecpnenet calculations (Dxc) with all their various coeeection and compenstion are employed to get some information on the pore pressure, Even on development wells, tha is to say in areas where much of the geology is no more secret, even thre pressure conditions may vary in an unforeseen way. The general pressure alert with its pit drills, regular checks, etc. is an ingredient indisspenable for any drilling operation.

The general pholosphy for drilling is to adjust the mud weight in such a way, that it compensates for the formation pore pressure, allowing a safety margin for p pressure when tripping out.If the mud weight is to high, the drilling progress is slowed down, or, in other words, the cos per foot drilled increase.

The ROP depends as well on the differential pressure acroos the bore bottom. If the mud weight is too high relative to the pore pressure, the cutting are held to the formation, the cleaning efficienty of the mud stream and is reduced, hence the drilling progress slowed down. Furthermore, the mud weight is too hight, the well is prone to lose fluids to the formation, thus inducing differential sticking of the drillstring and damage potential reservoirs. In constrast, if the mud weight is tooo low, the well is prone to kick , and unsafe drillihng practice.

It is outside the responsibility of the Wellsite geologist to fine the correction mud weight, In fact the geolgist and the drillign people ( person by the company man) may have a substanitail diffenece in understanding as to what the correct mud weight should be.

It is outside the responsibility of the sellsite geologist to find the correct mud weight. In fact the geologist and the drilling people (personified by the company man) may have substantial diffeence in understanding as to what the correct mud weight should be.

From the preprective of the mudlogging shack, proper mud weight id deduce for the folloing observations.

? Reasonable drilling progress (considering all the factores contributing to ROP such as

formation drillability, weight on bit, bit wear, etc.)

Distinct trip gas and connection gas peaks over gas background.If yo can see the trip gas and connection gas on the strip charts the mud weight appears to be correct. If the connection gas is higher than 50% of the background the mud weight may be on the low side.

? Normal shaped cutting with munimum (shale_) caving. In articular if the shale caving

increase in number and size, you may ba approachin a zone of overpressure or abnormal

pressure. Report to the company man>

The term abnormal pressure is used to descripe any situation of a pressure gradient deviating from hydrostatic pressure. However, it usually means overpressure, i.e., a pressure regime exceeding

hydrostatic pressure. Abnormal pressure may occure in young sediments, zones of rapid sementaion, young uplift and fast or deep burial. The advent of an overpressured zones will result in a change in a number of physical properties of the formation, which are refelected in the change of a number of paramertes, The following obervation, which are reflected in the change of a number of parameters. The following obersrvation might indicate a high pressure zones, however, almost all of them can have other reasons.

INDICATORS>

On incrasing pore pressure while drilling.

Gain in pit volume (KICK)

Increase in ROP (Drilling brak)

Increased torque while drilling.

Drag on trip and connections.

Bottom fill after trips (or wiper trips)

From Drilling returns.

Increased back ground gas, connection gas, (mud gas may show an increase in carbon dioxide).

? In some areas is has proven empirically true that an inversion of mud chromotograph gas

that is C3>C2> or C2>C1 indicate the approach to an overpressure zone. In those areas

where his relationship has been established,this indicator is fairly reliable.

? Change (increase) in flowline temperature. Only applicable, if the drilling is steady and the

temperature stabilized, otherwise external effects will cover the subtle change of flow line temperature.

Change of shape of cutting, typical pressure cavings. The shape pressure caving may not be

mistaken for swelling or sloughing claystone, This is the most sensitive and still only a quanlitative indication, Under overpressure conditions the shales (claystones) have a typical elongated shape, pressure caving similar shape and increase size.

Try to establish it the caving lithology comes frome the bottom of the hole (new formation) or if a formation drilled higher up caves in.

If you see shale slivers coming over the shaker screens, that are bigger than teeth of the drilling bit, the absolute alarm situation has been rached. Inform the company man..

INDICATORE CALCULATED ANALYZED AT THE WELLSITE

Shale density decreases or deviates for the trend line.

Water loss of shale cutting increases.

Significant increase in potassium content (mud, Mud filtrate and shale water.)

Shale water (filtrate) may have amber color.

Sharp change in cation exchange capacity (shale fatore measured by titration with methylene blue)

Decreasing D-Ecponent.

A number of empirical techniques have been developed to calculate pore pressure form wireline logs. The principle of most techniques is to establish a trend line of a parameter (logarithmic plot of resistivity, sonic travel time, density, neutron prorosity, ect) versus depth. Deviations form the trend line are interpreted to be indicative of abnormal pressures. These empirical work good in the areas where they were developed, Outside their classical application areas they are less correct, Though not necessarily wrong.

A lot has been published about overpressure detection from wireline and empirical formulae have been developed to estimate pore pressures from logs. In any case the drilling bit must have reached the zones of abnormal pressure and it must have been logged. Overpressure situations need fast answers because the drilling problems may already start after a few feet of cap rock drilled. Back to square one.

The situation is, however, vastly different if yo have MWD tools or real- time logging. Therefore , watch all drilling paramneters closely (in particular the mud pit level and the background gas) and try to come up with a sensible interpretation.

Another aspect of abnormal formation pressure are sub-normal pressures. Such situations are common when a wll is drilled through a formation that has been produced as a reservoir, initial reservoir pressure is reduced as oil, gas or water has been removed from the reservoir. This may lead to particularly problematical situations.

When drilling in producing oil fields where produced and unproduced reservoir horizons are close together. In this situation, a relatively high mud weight may be required to hold the fluids of the unproduced reservoir back in the formation while a relatively low mud weight may be ideally, these situationa are taken into account when designing a well. The well program then try to case-off one formation before drilling into the next formation that may have a different pressure regime.