Q921 de2 lec2 v1

Drilling Engineering 2 Course (1st Ed.)

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mailto:[email protected]

http://www.about.me/AlamiNia

1. General Notes

2. Pore Pressure Prediction

3. Abnormal vs. normal Pressure

4. Fracture Gradient determination

Introduction

The knowledge of the formations to penetrate, their strength properties as well as their behaviour when in contact with various drilling fluids is essential to properly plan and complete a successful drilling project. Parameters like pore pressure and formation strength determine aspects like:Choice of mud weight profile,Determination of casing setting depths,Design of optimal casing strings,Selection of the drill bit,Cementing additives and procedures.

2013H. AlamiNia Drilling Engineering 2 Course: Geomechanics 4

pressures as gradients

The way how the formations react to drilling mud influences the selection of mud additives, borehole stability and therefore well control aspects.

Within drilling, it is common to express pressures as gradients. With this concept, the hydrostatic pressure can be given

as equivalent density which is independent of the depth and thus makes its comprehension and correlations of various concepts easier.


reference depth

On the other hand, when gradients are applied, it has to be always kept in mind that they are referred to a specific depth. Knowing this reference depth is essential to compute

back the corresponding downhole pressures. Within drilling engineering, the drilling floor or rotary table

(RKB) is the most often used reference depth.

Geologists and geophysicists generally prefer to use their data in reference to ground floor or mean sea level (MSL).


Geology Prediction

Normally when a well is to be drilled, the drilling engineer is supplied from the geology department (or the geologist within the project team) with a sequence of predicted subsurface formations, their characteristics and markers, as well as knowledge about where special care has to be taken.

Geologists draw this information from studying the local geology (deposition history), seismic mappings (2D or 3D surveys) and perform well to well correlations (geological maps).

Whenever new information is gained (due to drilling and evaluation of a new well or further geophysical measurements) these maps are updated.


Typical geological profile to plan a well

Typical geological profile Seismic record to determine the subsurface structure2013H. AlamiNia Drilling Engineering 2 Course: Geomechanics 8

local subsurface pressure regimes

To understand the local subsurface pressure regimes, the geologic processes along with the depositional history and tectonic abnormalities have to be studied.

When the well is located within shallow sediments that were laid down slowly within a deltaic depositional environment, the subsurface formation pressures can be assumed to be hydrostatic.


Hydrostatic Pressure

By definition, a hydrostatic pressure is developed due to the own weight of a fluid at a certain depth. This relationship is expressed as: 𝑝 = 𝜌. 𝑔. ℎ = 9.81. 𝜌. ℎ

Or in field units:𝑝 = 0.052. 𝜌𝑓𝑙 . 𝐷

where:• 𝜌𝑓𝑙 [ppg] density of the fluid causing hydrostatic pressure

• 𝜌 [kg/m3] average fluid density• D [ft] depth at which hydrostatic pressure occurs (TVD)• h [m] vertical height of column of liquid• p [psi] hydrostatic pressure• g [m/s2] acceleration due to gravity


hydrostatically pressured formation

When the weight of the solid particles buried are supported by grain-to-grain contacts and the particles buried water has free hydraulic contact to the surface, the formation is considered as hydrostatically pressured.

As it can be seen, the formation pressure, when hydrostatically pressured, depends only on the density of the formation fluid (usually in the range of

1.00 [g/cm3] to 1.08 [g/cm3]) and

the depth in TVD.


overburden stress

When the burial depth increases, the overlaying pressure (overburden stress) increases. This decreases the pore space

between the grains and thus the porosity of the formation.

The overburden stress can be calculated assuming an average bulk density b of the overlaying formations as:

Porosity profile2013H. AlamiNia Drilling Engineering 2 Course: Geomechanics 13

average bulk density

The average bulk density is normally found by integration of the density log readings.

When density logs were not run (e.g. at shallow formations), sonic log correlation methods,

together with lithology and mineralogical evaluations are applied to determine 𝜌𝑏

During burial of the sediments, formation water is constantly expelled due to the reduction of formation porosity, as see in next slide.


Volume of fluid expelled during compaction of an argillaceous sediment


abnormally pressured

As long as formation water can be expelled, the formations are hydrostatic (or normally) pressured.

When drilling a well, formations are often encountered that are under a different pressure regime.These formations are named to be “abnormally

pressured”. Abnormal pressures can be positive

• (actual formation pressures are higher than hydrostatic pressure)

or negative “subnormal pressure”

• (actual formation pressures are lower than hydrostatic pressure).


Abnormally Mechanisms

Some mechanisms that lead to abnormally pressured formations are:1. Compaction effects,2. Aquathermal expansion,3. Diagenetic effects,4. Differential density effects (Osmosis),5. Fluid migration effects,6. Evaporite Deposits,7. Organic matter transformation,8. Tectonics,9. Connection to depleted reservoirs,10. Others.

From the various effects mentioned above, the compaction one is considered to be often the governing one.


normally pressured formations

while burying of the sediments, formation water is expelled with increasing depth and temperatures due to reduction in pore space and diagenesis of the rock materials.

As long as the permeability and the effective porosity of the rock is high enough so that the formation water can escape as quickly as the natural compaction takes place, the formations are normally pressured.


Modelling vertical Pressures

The (vertical) pressures acting inside formations can be modelled as: 𝜎𝑜𝑏 = 𝜎𝑧 + 𝑝where:

𝜎𝑜𝑏 [psi] overburden stress

𝜎𝑧 [psi] vertical stress supported by the grain-to-grain connections

p [psi] formation pore pressure


abnormally pressured formations

When the formation water can not escape as quickly as the pore space is reduced, it is trapped inside the formations. In this scenario, the increasing overburden stress will

pressurize the formation water and the formation will become abnormally pressured.

In this situation, the porosity of the formation will not follow the natural compaction trend (porosity at abnormally pressured formations will be higher than at normally pressured ones).

Along with the higher porosity, the bulk density as well as the formation resistivity will be lower at abnormally pressured formations.

These circumstances are often applied to detect and estimate the abnormal formation pressures.


formation pore pressures

The actual measurement of formation pore pressure is very expensive and possible only after the formations have been drilled.In this respect, pore pressures have to be estimated

before drilling to properly plan the mud weights,

casing setting depths,

casing design, etc.

as well as being closely monitored during drilling.


pore pressure estimation

To estimate the pore pressure and most important, define where abnormal pore pressures are to be expected, porosity logs and seismic measurements are applied most often.

shale formations tend to follow a defined porosity reduction trend with increasing depth. When this trend is interrupted, abnormally pressured

formations are to be expected. The knowledge of its depths are important since they may

lead to a necessary setting of casing and weighting up the mud system. The amount of how much the mud weight has to be increased

depends on the amount of abnormal pressure expected and the contingency of the well.


Abnormal pressure detectionwhile drillingWhen the well is in progress

and abnormal formation pressures are expected, various parameters are observed and cross-plotted. Some of these while drilling detection methods are: (a) Penetration rate, (b) “d” exponent, (c) Sigmalog, (d) Various drilling rate

normalisations, (e) Torque measurements, (f) Overpull and drag, (g) Hole fill, (h) Pit level – differential flow

– pump pressure,

(i) Measurements while drilling,

(j) Mud gas, (k) Mud density, (l) Mud temperature, (m) Mud resistivity, (n) Lithology, (o) Shale density, (p) Shale factor (CEC), (q) Shape, size and

abundance of cuttings, (r) Cuttings gas, (s) X-ray diffraction, (t) Oil show analyzer, (u) Nuclear magnetic

resonance.


Abnormal pressure evaluation

After an abnormal pressure is detected or the well is completed, various wireline log measurements are used to evaluate the amount of overpressures present. Among the most common ones are:(a) Resistivity, conductivity log,(b) Sonic log,(c) Density log,(d) Neutron porosity log,(e) Gamma ray, spectrometer,(f) Velocity survey or checkshot,(g) Vertical seismic profile.

With these log measurements trend lines are established and the amount the values deviate at the abnormally pressured formations from the trend line are applied to determine the value of overpressure.


Schematic responses of wireline logs in an undercompacted zone


Leak-off data

Normally, after a casing is set and cemented, a so called leak-off test (LOT) is performed. The main issue of a LOT is to check the strength of the

formation at the casing shoe.

With this knowledge, the maximum kick pressure allowed that does not fracture the formation is determined.

It is also the key parameter in stress modelling and borehole integrity evaluation.


formation integrity test

Sometimes the LOT test is not continued until leak-off (especially when oil based muds are used) and the formation is only pressured up until a certain value. This test is called formation integrity test (FIT).

In this way, when fracture strength is evaluated, it is important to distinguish LOT data and FIT data.


fracture gradient

The pressure where fractures are initiated is commonly called leak-off pressure and when referred to the individual depth, named fracture gradient.

The determination of fracture gradients for shallow depth is often difficult since very little data exists. This is due to the circumstance that at shallow depth,

blowout preventers are often not installed and thus no pressure testing can be carried out.

Especially at offshore wells, the knowledge of shallow fracture gradients are important since the margin between pore pressure and fracture gradient is narrow and the danger of shallow gas pockets exists.


1. Dipl.-Ing. Wolfgang F. Prassl. “Drilling Engineering.” Master of Petroleum Engineering. Curtin University of Technology, 2001. Chapter 3

Education

Q921 de2 lec2 v1