Excerpt
List of Contents
Abbreviations
1.Introduction
1.1 Objectives
2.Pore pressure
2.1 Estimating pore pressure
2.2 Pore pressure importance
2.3 Sources of pore pressure
2.4 Fracture pressure
2.5 Fracture gradient
3.Drilling
3.1 Rate of penetration
3.2 Rock mineralogy
3.3 Lithology study
3.4 Sandstone zone
3.5 Shale zone
3.6 Drilling problems
4.Casing program
4.1 Conductor casing
4.2 Surface casing
4.3 Intermediate casing
4.4 Production casing
4.5 Production tubing
4.6 Casing setting depth
5.Drilling mud
5.1Basic properties
5.2 Water base mud
5.3 Types of WBM
5.4 Oil base mud
6.1 Cement classification
6.2 Cement selection
References
List Figures
Figure 1 – Pressure graph
Figure 2 – Casing depth
List of Tables
Table 1 – Pressure formations
Table 2 – Geological prognosis
Table 3 – Pressure gradient
Table 4 – Casing size
Table 5 – Mud programme
Abbreviations
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1. Introduction
A new prospect is planned to be drilled on a new exploratory well at the Molly field. The field is located in the North Sea Block 14a/b. Kenmac
Petroleum Corporation has been assigned to carry out a number of tasks necessary as a part of an effective well planning. Feasibility studies based on pore and fracture will be evaluated in order to assess the possible zones of abnormal formations and the steps necessary to prevent drilling problems. Furthermore, a casing scheme will be proposed along with the mud programme and the cementation suitable for the particular well.
1.1. Objectives
- Drill vertically a new exploratory well
- Evaluate reliable data source for pore and fracture pressure
- To elaborate a casing scheme, a mud programme and the
Cementation
2. Pore Pressure
Pore pressure is the pressure exerted by the fluids contained in the pore space of a rock. It is normally related with the fluid column density and vertical depth. Pore pressure data can be predicted utilizing (1) seismic interval velocities, and (2) offset well logs (Selim, Badway and Abdullah,
2010). Wildcat well planning, often uses the predictive method to estimate the formation pressure. Predictive methods are primarily based on (1) data available from nearby wells and (2) seismic data.
2.1. Estimating Pore Pressure
Two basic approaches are used to make a quantitative estimate of formation pressure (Bourgoyne et al, 1991, p. 253)
- Plots of a porosity-dependent parameter vs. depth – assuming that the effective stress matrix throughout an abnormally pressured formation as well in normally shallow pressured formation, are the same.
- Plots of a porosity-dependent parameter vs. depth using empirical correlations.
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Fig. 1 – Normal and Abnormal pressure graphs (Adapted from Bourgoyne et al, p.254)
Formation pore pressure can be normally or abnormally pressured. The normal pore pressure is the hydrostatic pressure due to the average density and vertical depth of the column of fluids above a particular point (Bera, 2010, p.1). As such pressures higher than normal are termed geopressured or over pressured and pressures lower than normal are termed subnormal, table below gives some typical values.
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Table 1 – Typical values for normal and abnormal pressure formations
Bourgoyne et al (1991) have clearly summarized four mechanisms for abnormal pore pressures, and these are:
- Compaction
- Diagenesis
- Differential density and
- Fluid migration
The most common abnormal pore pressure generating mechanism is the compaction.
2.2. Formation Pore Pressure - Importance
The prediction of pore pressure is of paramount importance in the well planning, because it helps to maximize the drilling safety, the borehole stability, the rig selection (Bera, 2010), anticipate the location of potential abnormal pressure formation zones, as well as helps to minimize the drilling cost, so that the mud density can be optimized to provide sufficient overbalance and assure the casing selection with depth, will withstand the various formation pressures (Selim, Badway and Abdullah, 2010). The wellbore pressure should always as reasonable as possible be kept at a pressure higher than the formation pressure. The wellbore pressure scenario mostly desirable when planning a well is the overbalanced pressure. The aim is to prevent the influx of formation fluids into the wellbore and avoid possible kicks.
2.3. Sources of Pore Pressure
When formation pressures are normal, the porosity dependent parameter should have an easily recognized trend, because of the decreased porosity with increased depth of burial and compaction. A departure from the normal pressure trend signals into a probable transition to abnormal pressure is very crucial for the well planning and drilling operations, since the depth at which the departure occurs is critical because the casing must be set in the well before excessively pressured permeable zones can be drilled safely (Bourgoyne et al, 1991, p.253).
Table 2- Molly Field – Geology Prognosis
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The prognosis depths and pore pressures for the reservoir formations are listed in the table below.
Table 3 – Pressure Gradient versus Depth
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2.4. Fracture Pressure
Fracture pressure is the critical pressure required to break down the formation or induce fractures. For well planning fracture gradient is used as a plot of pressure vs. depth to estimate when a rock will fail and induce fractures.
Formation fracture data are estimated by predictive methods. Since formation fracture pressure is affected greatly by the formation pore pressure (Bourgoyne et al, 1991), the methods of correlation applied in pore pressure prediction, must be applied before using a fracture pressure correlation. Fracture gradient helps to determine the setting depths for intermediate casing strings, the maximum allowable annular surface pressure allowed to control a kick, and maximum allowable mud density for drilling.
2.5. Methods for Estimating Fracture Gradient
Fracture pressure is estimated by using equations and correlations.
Equations are from Hubbert and Willis, and Christman. The correlations are from Mathew and Kelly, Penabaker, Macpherson and Berry, and Eaton.
3. Drilling
Several problems are encountered while drilling through different formations. A study of lithology column helps to predict the possible zones of drilling problems and as so the measures to deal with those problems. The following paragraphs explain the background of drilling through different formations.
During drilling operations, the bottomhole pressure should be maintained slightly higher than the formation pore pressure. The confinement associated with this static differential pressure causes an increase in the drilling strength of rock. During drilling, the rock ahead of the cutter is rapidly deformed. The rock deformation associated with the dilatation leads to a dynamic pressure differential between the borehole fluid and pore fluids of the rock being cut that can equal the total borehole pressure. Dynamic confinement pressure increases the strength and plasticity of rock, reducing the efficiency of indentation and shear cutting
(Kolle., 2000. p. 1).
3.1. Rate of Penetration
The rate of penetration is considered as one of primary factors which affect the drilling efficiency through different lithology formations. As such bit torque is used to classify lithology into three categories, (1) porous, (2) argillaceous (shaly), and (3) tight (Wolcott and Bordelon, 1993, p.769).
Porous rock e.g. sandstones are weaker than argillaceous rocks (shales), which in turn are weaker than tight rocks (Falconer, Burgess and Sheppard, 1998 p.124).
The problems that may be encountered while drilling through different formations are related to a transition between normal pressure zones to over pressured zones. The rate of penetration is strongly affected by some important variables such as (1) bit type, (2) formation characteristics, (3) drilling fluid properties, (4) bit operating conditions (bit weight and rotary speed), (5) bit tooth wear and (6) bit hydraulics.
3.2. Rock Mineralogy
The mineralogy of the rock has some effect on penetration rate. Rocks containing hard, abrasive minerals can cause rapid dulling of the bit teeth. Rocks containing gummy clay minerals can cause the bit to ball up and drill in a very inefficient manner.
3.3. Lithology Study
The Molly Field lithology column is characterized by sandstones, shales and clay.
3.4. Sandstone zone
Drilling through hard sandstone poses particular problems for bit optimization. Conventional methods for raising rate of penetration ROP, such as use of PDC bits, are limited by rapid cutter wear in hard, quartz rich formations. In these non hydratable formations, bit cleaning may also exert a negligible effect on rate of penetration ROP, closing off another potential source of improvement. In practice this means that options to raise rate of penetration ROP in hard sandstone may be restricted to exploiting the effects of drilling parameters, with bit types capable of resisting rapid abrasive wear (Fear, 1999, p. 46)
3.5. Shale zone
When drilling through shales formations the two major problems encountered are the shale sloughing and swelling. Shale instability in a wellbore is attributed to any of the following combination of forces: (i) overburden pressure, (ii) degree of compaction at the formation, (iii) pore pressure in shale exceeding the hydrostatic pressure (iv) tectonic forces and (v) presence of micro fractures along cleavage planes on the clay platelets (Talabani, Chukwu and Hatzignatiou, 1993, p.283).
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- Quote paper
- Ataliba Miguel (Author), 2012, Well Planning at Molly Field, Munich, GRIN Verlag, https://www.grin.com/document/278037
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