V. Sigitov, D.Batyrbekuly

Kazakh-British Technical University

ESTIMATION OF POSTTREATMENT SKIN FACTOR VALUE

AT KUMKOL OILFIELD

In the article below, it was explained the primary goal of well stimulation, which is to increase the productivity of a well by removing damage in the vicinity of the wellbore or by superimposing a highly conductive structure on to the formation. New and novel fracture stimulation technologies can provide attractive deliverability enhancement results. Even though fracture stimulation was very successful at several test sites and led to significant improvements in deliverability, not every stimulation treatment was effective. The most consistently effective stimulation technology should be hydraulic fracturing.

In the paper, we tried to pay your attention to Hydraulic fracturing, the stimulation method which is intended to provide a net increase in the productivity index, that can be used either to increase the production rate or to decrease the drawdown pressure differential. A decrease in drawdown can help prevent sand production and water coning, and shift the phase equilibrium in the near well zone toward smaller fractions of condensate. Injection wells also benefit from stimulation in a similar manner. Commonly used stimulation techniques except hydraulic fracturing, include fracpack, carbonate and sandstone matrix stimulation (primarily acidizing), and fracture acidizing.

Many reservoirs must be hydraulically fractured to become economically productive.

Hydraulic fracturing involves injecting a large volume of proppant-laden fluid at a pressure sufficiently high to fracture the formation. After the fracturing fluid leaks off into the formation, the remaining proppant keeps the fracture open. Although a hydraulic fracture is narrow (a fraction of an inch in most cases), the presence of this high-permeability channel significantly enhances the productivity of the well. The presence of a fracture alters the flow regime inside the formation as the fluid flows into the fracture and then through the fracture into the wellbore, with very little or no fluid flowing directly from the formation into the wellbore. The presence of a hydraulic fracture adds another dimension to the fluid flow in porous media and to well test design and analysis.

Using of hydraulic fracturing at KUMKOL oilfield recommends starting at J-II horizon, with installation of packer against clay in the interval: 1300.0-1305.0 m.

Necessary measures before doing hydraulic fracturing are:

- Isolation of interval 1312.0-1316.0 m, at the bottom, where it was fluid flow

with water (December, 2001)

- Installation of cement plug at the depth of 1317.0 m in order to avoid

deformation of casing against water producing formation of J-III horizon.

- Re-perforation filter 1305.6-1308.4m

Figure – 1.Pressure distribution in a reservoir with a skin

It was proven that as a result of drilling and completion practices, the formation

permeability near the wellbore are usually reduced. Drilling fluid invasion of the formation, dispersion of clay, presence of a mudcake and cement tend to reduce the formation permeability around the wellbore. A similar effect can be produced by a decrease in the area of flow exposed to the wellbore. Therefore, partial well penetration, limited perforation, or plugging of perforations would also give the impression of a damaged formation. Conversely, an inclined well or inclined formation increases the area of flow near the wellbore, giving the impression of a stimulated well. The zone of reduced (or higher) formation permeability has been called a ‘‘skin,’’ and the resulting effect on well performance is called ‘‘skin factor.’’ Skin factor can be used as a relative index to determine the efficiency of drilling and completion practices. It is positive for a damaged well, negative for a stimulated well, and zero for an unchanged well (Figure-1). Acidized wells usually show a negative skin. Hydraulically fractured wells show negative values of skin factor that maybe as low as -7. Well stimulation ranks second only to reservoir description and evaluation as a topic of research or publication within the well construction process. There as, on for this intense focus is simple: this operation increases the production of petroleum from the reservoir. Thus, this facet of the construction process actively and positively affects a reservoir’s productivity, where as most of the other operations in this process are aimed at minimizing reservoir damage or eliminating production problems.

Hydraulic fracturing is a proven technological advancement which allows natural gas

producers to safely recover natural gas from deep shale formations. This has the potential to not only dramatically reduce our reliance on foreign fuel imports, but also to significantly reduce our national carbon dioxide (CO2) emissions and to accelerate our transition to a carbon-light environment. Experts have known for years that natural gas deposits existed in deep shale formations, but until recently the vast quantities of natural gas in these formations were not thought to be recoverable. Today, through the use of hydraulic fracturing, combined with sophisticated horizontal drilling, extraordinary amounts of natural gas from deep shale formations across the United States are being safely produced. Hydraulic fracturing has been used by the oil and gas industry since the 1940s and has become a key element of natural gas development worldwide. In fact, this process is used in nearly all natural gas wells drilled in the U.S. today. That’s why we should use this method almost in all oil and gas wells in Kazakhstan. Properly conducted modern hydraulic fracturing is a safe, sophisticated, highly engineered and controlled procedure.

KEY POINTS:

• Hydraulic fracturing is essential for the production of natural gas from shale formations.

• Fracturing fluids are comprised of more than 99% water and sand and are handled in

self contained systems.

• Freshwater aquifers are protected by multiple layers of protective steel casing

surrounded by cement; this is administered and enforced under state regulations.

• Deep shale gas formations exist many thousands of feet underground.

Fracturing Fluid Makeup

In addition to water and sand, other additives are used in fracturing fluids to allow

fracturing to be performed in a safe and effective manner. Additives used in hydraulic

fracturing fluids include a number of compounds found in common consumer products.

Hydraulic fracturing consists of injecting fluid into the formation with such pressure that it induces the parting of the formation. Proppants are used in hydraulic fracturing to prop or hold open the created fracture after the hydraulic pressure used to generate the fracture has been relieved. The fracture filled with proppant creates a narrow but very conductive path towards the wellbore. In almost all cases, the overwhelming part of the production comes into the wellbore through the fracture; therefore, the originally present near-wellbore damage is ‘‘bypassed,’’ and the pretreatment positive skin does not affect the performance of the fractured well. Perhaps the best single variable to characterize the size of a fracturing treatment is the amount of proppant placed into the formation. Obviously, more propped fracture volume increases the performance better than less, if placed in the right location. In accordance with the general sizing approach outlined above, the final decision on the size of the fracturing treatment should be made based on the NPV analysis.

Often the well is cased a steel casing cemented to the rock. The pressurized fluid that causes cracking must be allowed to pass from the cased well bore to the rock. This is done by perforating the casing and cement. The process of perforating the casing causes damage to the rock as well as the cement bond between the steel and the rock. Fluid may be able to enter the interface between these materials and fracture initiation may not occur from the perforations as desired.

Finally, the wells may not always be vertical and the fractures may not always be simple planar features. Multiple deviated wells are commonly drilled from ocean drilling platforms,and horizontal wells are increasing. Fractures from these various wells, in addition to vertical wells, need to be modeled.

Example of Typical Deep Shale Fracturing Mixture Makeup

A representation showing the percent by volume composition of typical deep shale gas hydraulic fracture components (see graphic) reveals that more than 99% of the fracturing mixture is comprised of freshwater and sand. This mixture is injected into deep shale gas formations and is typically confined by many thousands of feet of rock layers.

To sum it up, engineers design a fracturing operation based on the unique characteristics of the formation and reservoir. Basic components of the fracturing design include the injection pressure, and the types and volumes of materials (e.g., chemicals, fluids, gases, and proppants) needed to achieve the desired stimulation of the formation.

When applied to stimulation of water injection wells, or oil/gas wells, the objective of hydraulic fracturing is to increase the amount of exposure a well has to the surrounding

formation and to provide a conductive channel through which the fluid can flow easily to the well. Computer models are used to simulate fracture pathways, but the few experiments in which fractures have been exposed through coring or mining have shown that hydraulic fractures can behave much differently than predicted by models.

References:

1. Economides, M.J., and Nolte, K. (eds.), Reservoir Stimulation (2nded.), Prentice Hall,

2. Englewood Cliffs, NJ (1989).

3. Haimson, B.C., and Fairhurst, C.:‘‘Initiation and Extension of Hydraulic Fractures in

4. Rocks,’’ SPEJ (Sept.1967)310–318.

5. "Application of New and Novel Fracture Stimulation Technologies to Enhance the

6. Deliverability of Gas Storage Wells," Topical Report, Contract No. DE-AC21-94MC31112, U.S. Department of Energy, Washington, DC (April, 1995).

7. 4. Gidley, J.L., Holditch, S.A., Nierode, D.E., and Veatch, R.W., Jr. (eds.), Recent

8. Advances in Hydraulic Fracturing, Richardson, TX (1989), SPEMonograph12.