Improved Model For Prediction And Remediation Of Formation Damage In Oil Wells

Introduction

Formation Damage

Formation damage can be defined as the reduction in the original permeability of the reservoir rock close to the wellbore. The potential of the near wellbore formation permeability being reduced (damaged) exits from the minutes the drilling bits enters the formation until the wells is abandoned.

From the time the drilling bit enters the pay zone until the well is put on production, the zone is presented to drilling fluids that could be detrimental to the future productivity of the well. At the point when drilling through the zone, the nature of the drilling liquid and the pressure differential are basic.

Generally oil and gas well reservoirs are penetrated using water based drilling fluids. The presentation of both the mud solids and polymers into the formation influences the liquid saturation in the pore space. The response between the formation fines and/or contradictory reservoir brine with moved mud fines and filtrates brings about a decrease of the formation original permeability. Oil wells that are completed in an open opening system, even generally shallow attack close to the wellbore might considerably hinder the stream in light of the fact that reservoir fluids must go through the damaged zones before production.

The formation can be damaged in distinctive ways. Physically, the formation can be damaged by:

The attacked mud solids obstructing the pore channels

The narrowing of vessels because of adsorption of attacked polymers furthermore

Water block, emulsion block and gas block.

Synthetically, the formation can be damaged by the response between the filtrate and pore substance and/or network materials. Swelling and scattering of muds and precipitation by the response between mud filtrate and pore content and additionally arrangement of salt and minerals from the network are the primary elements.

Bacterially, the formation can be damaged by the settlement of microscopic organisms and their hastened items, obstructing the pore channels.

In this manner with a specific end goal to augment the economic benefits of oil wells, it is important to know the attributes of damage accounted during drilling and to create strategies to minimize the degrees of damage brought on by drilling fluids.

Van Everdingen and Hurst presentation the ideas of skin component to the petroleum business. They perceived that for a given flow rate, the measured bottom hole flowing pressure was not as much as that computed hypothetically. This demonstrated that there was an extra pressure drop over that figured hypothetically and is free of time. They ascribed this pressure drop to a little zone of changed or decreased permeability around the wellbore and called this “attacked/damaged/skin” zone. They suspected that attacked zone is because of reservoir pollution by mud and stopping of some pore spaces around the wellbore. Numerically, skin drop is exhibited by,

………..1.1

The concept of thin skin in the above equation works very well in damaged wells. But because of mathematical and physical difficulties when the well is stimulated i.e negative skin, it has to be generalized.

Hawkins modified the above equation by introducing the concept of thin skin. He defined skin factor for a damaged zone of radius rs with permeability ks in a formation with permeability k and wellbore radius rw 9see figure 1.2 & 2.2) as:

………..1.2

Theoretically, the skin factor for a damaged well can vary from zero to and for simulated well, the skin can vary from zero to a value as low as -6.

1.1 Skin Damage & Formation Permeability

Skin damage is brought about by drilling liquid attack, which decreases the permeability around the wellbore. A high permeability reservoir exhibits a high attacked zone than a low permeability reservoir. However the rate loss in permeability in a high permeability zone is smaller than that in a low permeability zone. This is on the grounds that those high permeability formations have extensive pore throat sizes, which are not totally obstructed by solids in drilling fluids.

Though pore throats or low permeability formations are little, mud solids and filtrates presumably obstruct those throats bringing about huge decrease in permeability. Comparison 1.2 demonstrates that change in permeability is more essential than thickness of the attacked zone.

FIGURE 1.1: A Schematic Of A Well With Damaged Zone (Skin Damage)

FOR A GIVEN RATE, q

Pwf1 > pwf > pwf2

FIGURE 1.2: Effect Of Skin Factor On Well Flow Pressure.

1.2 Objectives / Aims of Study

The objective of this project is

To recognize, diagnose the causes of formation damage.

To analyze the effect of the damage to the wells.

To evaluate the economic effects of formation damage.

To make recommendation of formation damage removal base on evaluation and analysis of data.

1.3 Scope of the Study

The study is concerned with the causes, effect on the wells and the economic effects of this damage on the wells as well as solutions of formation damage in oil wells.

1.4 Brief History of Well X

This well was drilled as a deviated appraisal well in 2003. It was also completed in 2004 as a TSM. It came on stream in June 2004 and was rapidly beamed up to potential. Production picked in January 2005 at 3702 BOPD on 44/46”. Interval started cutting water in November 2006, thereafter, production started to decline. Further beam-ups failed to increase production or arrest decline. An abnormal decline in FTHP occurred in June 2007 following a beam up to 52/64”. Since then THP has been fairly low. The production fell to 1250 barrels per day. A stimulation program was carried out in June 2008 when the production rate decreased to 975 BOPD.

1.5 Methodology

This method adopted in writing this research project is through abstracts obtained from journals, textbooks, SPE technical papers and presentations. The research project also involved the receiving of past project of similar subject matters and obtaining relevant information.