Damage and Stimulation

Introduction and Darcys Law

PETE 4052Well TestingSpring 2002

Lecture 4Damage and Stimulation: The Skin EffectFebruary 4

Damage and Stimulation: The Skin Effect

Outline

0. Synopsis

1. Description of Damage and Stimulation

2. A Radial Composite Model for Damage and Stimulation

3. A Lumped Model for Damage and Simulation: Skin Effect

4. Inflow Equation Including Skin

5. Discussion

Related reading

Dake: p 115-121.

Horne: Section 2.3, p. 12-16 (attached)

4.0 Synopsis

We have discussed radial and linear flow in composite systems in Lecture 3. When the region of alteration is very small, it may be necessary and useful to idealize the altered region as a zero-thickness skin. Skins are used to describe both damage and stimulation.

Key concepts:

Representation of damage or stimulation as an altered region around the wellbore

Pressure drop across the altered zone

Idealization of the altered zone as a zero-thickness skin

Definition of skin

Dimensionless pressure

Flow efficiency

Apparent wellbore radius

4.1 Description of Damage and Stimulation

The processes of drilling, completing and producing an oil or gas well include many mechanical, hydraulic, and chemical processes. Many wells are drilled overbalanced, so that drilling fluids migrate into the near-well area. The fine particles in the muds may plug pore throats, or the filtrate may react chemically with clays in the formation either of these processes can reduce the near-well permeability dramatically. Completions may further reduce the productive capacity of the well: the well may be cased and perforated (reducing the inflow area compared to an open-hole completion), partially penetrating (reduced thickness for inflow), or internal gravel-packed (pressure loss through perforations, gravel, and screen). On the other hand, the pressure-drop in the near-well area can sometimes be increased. This could be accomplished by fracture treatments or acid treatments. Attempts to lower the pressure drop in the near-well area are often called stimulation.

We will present some sketches of these situations in the lecture.

4.2 A Radial Composite Model for Damage and Stimulation

The simplest model for wellbore alteration is a radial composite model. The permeability is assumed to be altered from the formation permeability,, to the altered or skin permeability, , in a region (Fig. 4.1). For situations with ks < k (damage), this results in an increased pressure drop, . Note that is NOT the pressure drop across the skin region! It is the change in pressure drop for the composite model (the model with altered permeability) compared with the model with no altered permeability region. For damage, is positive whereas is negative for stimulation. I guarantee some students will confuse with . Look at Figure 4.1, and make certain that you understand the difference!

We can now write expressions for the pressure drops using our knowledge of the radial flow equation and series flow. The pressure drop across the skin region is

(4.1)

Note that this pressure drop increases as rs increases and ks decreases: this makes senses, because thicker, lower permeability skin zones cause more pressure drop, and more damage. If the permeability had not been altered, the pressure drop would be

(4.2)

Combining Eqns. (4.1) and (4.2), we get an equation for :

(4.3)

Examine the term . Note that will be positive when k>ks, and negative when k