Бурение грунтовых зондов, установка энергетических колодцев

General:

• Define the terms: Pressure Gradient; hydrostatic pressure; “Normal” Pressure; “Abnormal” Pressure; Overburden(geostatic) Pressure; Fracture pressure.

• Plot the above from a set of data from a well.

• Describe in general terms the origins and mechanisms which generate Overpressured and Underpressured reservoirs

• Describe in detail the mechanism of Undercompaction

• Describe the characteristics of the different types of seal above an abnormally pressured formation and their implications for overpressure detection.

• Describe the impact of Abnormally pressured formations on well design and drilling operations

Overpressure Prediction and Detection Techniques:

• List and describe the methods of predicting overpressures before drilling the well. Prioritise these techniques in order of reliability in a given environment.

• List and describe the techniques used for the detection of overpressures whilst drilling a well.

• Define the assumptions inherent in, and limitations of, the “d” exponent technique for overpressure detection.

• Calculate and plot the “d” exponent and modified “d” exponent and use these to determine the top of the overpressured zone.

• Calculate and plot shale density when used in the determination of overpressures.

Leak Off Test and Fracture Pressure:

• Describe the mechanisms of formation breakdown

Define the terms: Limit test and Leak off test.

Describe the procedure used when conducting a leak off test.

Calculate the: maximum allowable mudweight (including ECD); and MAASP for the subsequent hole section after conducting a LOT.

1. INTRODUCTION

The magnitude of the pressure in the pores of a formation, known as the formation pore pressure (or simply formation pressure), is an important consideration in many aspects of well planning and operations. It will influence the casing design and mud weight selection and will increase the chances of stuck pipe and well control problems. It is particularly important to be able to predict and detect high pressure zones, where there is the risk of a blow-out.

In addition to predicting the pore pressure in a formation it is also very important to be able to predict the pressure at which the rocks will fracture. These fractures can result in losses of large volumes of drilling fluids and, in the case of an influx from a shallow formation, fluids flowing along the fractures all the way to surface, potentially causing a blowout.

When the pore pressure and fracture pressure for all of the formations to be penetrated have been predicted the well will be designed, and the operation conducted, such that the pressures in the borehole neither exceed the fracture pressure, nor fall below the pore pressure in the formations being drilled.

2. FORMATION PORE PRESSURES

During a period of erosion and sedimentation, grains of sediment are continuously building up on top of each other, generally in a water filled environment. As the thickness of the layer of sediment increases, the grains of the sediment are packed closer together, and some of the water is expelled from the pore spaces. However, if the pore throats through the sediment are interconnecting all the way to surface the pressure of the fluid at any depth in the sediment will be same as that which would be found in a simple colom of fluid. The pressure in the fluid in the pores of the sediment will only be dependent on the density of the fluid in the pore space and the depth of the pressure measurement (equal to the height of the colom of liquid). it will be independent of the pore size or pore throat geometry. The pressure of the fluid in the pore space (the pore pressure) can be measured and plotted against depth as shown in Figure 1. This type of diagram is known as a P-Z diagram

The pressure in the formations to be drilled is often expressed in terms of a pressure gradient. This gradient is derived from a line passing through a particular formation pore pressure and a datum point at surface and is known as the pore pressure gradient. The reasons for this will become apparent subsequently. The datum which is generally used during drilling operations is the drillfloor elevation but a more general datum level, used almost universally, is Mean Sea Level, MSL. When the pore throats through the sediment are interconnecting, the pressure of the fluid at any depth in the sediment will be same as that which would be found in a simple colom of fluid and therefore the pore pressure gradient is a straight line as shown in Figure 1. The gradient of the line is a representation of the density of the fluid. Hence the density of the fluid in the pore space is often expressed in units of psi/ft.

Geological

Pressure, psi

Figure 1 P-Z Diagram representing pore pressures

Pressure ( / 4 Guage I J

Section

This is a very convenient unit of representation since the pore pressure for any given formation can easily be deduced from the pore pressure gradient if the vertical depth of the formation is known. Representing the pore pressures in the formations in terms of pore pressure gradients is also convenient when computing the density of the drilling fluid that will be required to drill through the formations in question. If the density of the drilling fluid in the wellbore is also expressed in units of psi/ft then the pressure at all points in the wellbore can be compared with the pore pressures to ensure that the pressure in the wellbore exceeds the pore pressure. The differential between the mud pressure and the pore pressure at any given depth is known as the overbalance pressure at that depth (Figure 2). If the mud pressure is less than the pore pressure then the differential is known as the underbalance pressure. It will be seen below that the fracture pressure gradient of the formations is also expressed in units of psi/ft.

Pressure, psi Figure 2 Mud density compared to pore pressure gradient

Most of the fluids found in the pore space of sedimentary formations contain a proportion of salt and are known as brines. The dissolved salt content may vary from 0 to over 200,000 ppm. Correspondingly, the pore pressure gradient ranges from 0.433 psi/ft (pure water) to about 0.50 psi/ft. In most geographical areas the pore 4

pressure gradient is approximately 0.465 psi/ft (assumes 80,000 ppm salt content) and this pressure gradient has been defined as the normal pressure gradient. Any formation pressure above or below the points defined by this gradient are called abnormal pressures (Figure 3). The mechanisms by which these abnormal pressures can be generated will be discussed below. When the pore fluids are normally pressured the formation pore pressure is also said to be hydrostatic.

0 2000 4000 6000 8000 10000 12000 Estimated Formation Pressure, psi

Figure 3 Abnormal formation pressures plotted against depth for 100 US wells

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