Class Diagrams

Stimulation - High Level Model

The aim of a stimulation activity is to improve the productivity or injectivity index that is deemed inadequate for an existing or new well. They are designed to increase the recovery rate of reserves but not the recoverable reserves themselves, although they can make production of a given well profitable when it would otherwise have an insufficient flow rate. Before undertaking a stimulation treatment, it is essential to identify the problem properly in order to choose the type of treatment that can actually solve it.

The term well stimulation encompasses several techniques used to enlarge old channels or to create new ones in the producing formation of a well. Since oil usually exists in the pores of sandstone or the cracks of limestone formations, enlarging or creating new channels causes the oil or gas to move more readily to a well. Sometimes the problem is low permeability. In this case, the well will be stimulated immediately after completion. In other cases, the natural permeability of the rock may be adequate, but the formation near the wellbore may be damaged in a way that restricts the flow channels in porous rack. Formation damage can occur during drilling, completion, workover, production, or injection.

There are three ways to do this. The first and oldest method is to use explosive fracturing. The second stimulation method is to use acid stimulation (acidizing), and the third stimulation method is hydraulic fracturing.

1) Explosives - nitroglycerine is exposed inside wells to improve their productivity. Nitro charge is simply lowered into the open hold on a conductor line and detonated in to fracture the formation. This method of stimulation was very popular until acidizing and hydraulic fracturing were discovered.

2) Acidizing - an acid reacts chemically with the rock to dissolve it. This enlarges existing flow channels and opens new ones to the wellbore. Well servicing crews stimulate both old and new wells with acid. Reservoir rocks most commonly acidized are limestone and dolomite. There are many different types of acids that can be used to stimulate the rock. Some acids are strong enough to dissolve not only rock, but eat away the metal of the pipes and equipment in the well. Therefore, acidizing always involves a compromise between acid strength and additives to prevent damage to the equipment. Additives are used with oilfield acids to prevent or delay corrosion; to inhibit the acid from attacking the steel tubing or casing in the well.

3) Hydraulic Fracturing - hydraulic fracturing is all about pressure. Several powerful pumps inject a liquid, the fracturing fluid, into the well at a fast rate. The fluid develops a high pressure that actually splits, or fractures the rock. In fracturing, the fluid acts as a wedge, and its high pressure is the force that pushes it into the rock. Hydraulic fracturing splits the rock instead of the casing because the casing is stronger than the rock. Hydraulic fracturing improves the productivity of a well by either creating new fractures that act as flow channels or extending existing flow channels farther into the formation. Fracturing is a usual part of completion, and refracturing to restore productivity of an old well is a regular procedure.

Hydraulically formed fractures tend to heal, or lose their fluid-carrying capacity, after the parting pressure is released unless the fracture is propped open in some manner. Proppants, or propping agents, hold the fractures open. Sand, nutshells, and beads of aluminum, glass, and plastic may be used as proppants. Spacer materials are used between the particles of the proppant to ensure its optimum distribution.

The fracturing fluid may be either oil based or water based. In reality, the fluid is nearly always brine because it is safe, available, and cheap. Some fracturing fluids are gels, which suspend the proppants better. Polymer additives reduce friction between the fluid and the walls of the tubing. Additives reduce fluid loss into the formation.

The following rules describe how to use the Stimulation classes:

 The high-level information about a stimulation activity will be stored in the Stimulation class.

 Information about each of the stages of a stimulation activity will be stored in the Stimulation_Stage class.

 The results of tests performed after the stimulation or stimulation stage activity is completed will be stored in the Stimulation_Evaluation class.

• The individual materials that are used during a stimulation activity are stored in the Fluid_Pumping_Detail class.