Improved Model for Prediction and Remediation of Formation Damage in Oil Wells
Chapter One
OBJECTIVES/AIMS OF STUDY
The objective of this project is
- To recognize, and 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 recommendations for formation damage removal based on evaluation and analysis of data.
CHAPTER TWO
LITERATURE REVIEW
Ever since Everdingen & Hurst introduced the idea of a skin factor to the petroleum industry in 1953, the primary research focus has been on evaluation and maximizing the formation impairment. The impact and importance of this skin factor to the economical production of crude oil seriously engaged the attention of many researchers in the last five decades. In these efforts, the effects of mud characteristics, reservoir behaviour drilling & work over operation conditions on the formation damage have been studied with physical, analogue, analytical and numerical simulation models.
To achieve high production rate and avoid gas and water, horizontal wells getting preference over their counterpart vertical wells. But most of the horizontal wells are completed open hole and are more likely to face this damaged zone formed during drilling operation around wellbore where a cased vertical well overcome it by perforation. In this regard, the influence of skin on the performance of horizontal wells for various reservoir, production and drilling operation conditions have been studied widely in recent years. These research studies investigated many parameters involved during drilling and production conditions by experimental, analytical and numerical techniques. A brief review of literature is presented below focusing the formation damage research studies induced during drilling operation.
Abram (1977) emphasized the addition bridging material to the drilling mud to minimize solids invasion and formation impairment. He defined two rules for selecting the size and concentration of the bridging additives.
- The median pore size of the bridging additives should be equal to or slightly greater than 1/3 the median pore size of the formation.
- The concentration of the bridging solids must be at least 5% by volume of the solid in the final mud mixture pressure and mud (water based) rate of 2-3 ft/min for 20 minutes.
- 5-50md Dolomite cores in linear model.
Experimental results with component mud showed that impairment caused by invasion of mud particles occurred to a depth of less than 1 inch. Studied systems has no significant effect on this impairment on productivity and infectively because it was easily penetrated by perforation.
Gruesbeck & Collin (1982) described the result of an experimental study performed to determine the factors affecting entrainment and re-deposition of naturally occurring fine particles in porous media, leading to abnormal decline in productivity of the wells.
Authors proposed a phenomenological theory of entertainment and deposition to determine local laws of deposition and entrainment. The central concept of this theory was representation of both particle and pore size distribution by partitioning the porous medium at any cross section into parallel plugging and non-plugging pathways.
The result showed a minimum interstitial fluid velocity for fine entrainment is required, which depends on properties of the porous medium and contained fluids. Transition from single to two-phase flow can result in a reduced critical velocity and enhance entrainment.
CHAPTER THREE
METHODOLOGY
BHP SURVEY
Pressure build up survey are functionally run into an interval to help diagnose the well’s problem. This can help to establish if the problem is reservoir related. Static BHP test is employed in the evaluation of the reservoir. If the pressure is lower than anticipated, the reservoir may be depleted. If the pressure is constant, there may be problems of restricted flow. If the BHP survey signals the problem is restricted flow (feed-in problems), it would be determined if it is attributed to damage or poor perforation and this forms the basis for the next remedial decision.
CHAPTER FOUR
ANALYSIS OF RESULT
The results obtained from the work showed that there is formation damage during production.
In finding out the major causes of formation damage in well X, it was observed that the damage was due to fines migration which occurs when the production rate was sufficiently high to dislodge the particles and allow them to flow towards the well bore and plug the pore throats. This shows that the damage in the two wells occurred after completion.
CHAPTER FIVE
CONCLUSION
Taking a critical analysis of formation damage in oil wells, it has been discovered that one of the major problems the engineers have is to ensure that formation damage is greatly reduced for effective production.
In conclusion, one can say that the major causes of formation damage in well X were as a result of movement of fines due to high production rate that has plugged the throat of the pores. This increases the permeability, flow efficiency, in flow performance and high increase in production of the well shows that the stimulation job embarked upon in the well was worked out using appropriate program. It will be correct to say that the major aim of the project was successful.
RECOMENDATION
Generally, from finding, it was made to understand that formation damage can occur in oil wells operations ranging from drilling to production.
Based on the results from well X, i recommend that the production rate should be reduced in order to reduce the migration of fines so as to prevent the occurrence of formation damage. Also, production rate should be controllably low at the earlier life of a well and only gradually increased to check sand/fines migration.
In addition, the following are recommended for effective diagnosing the causes and to achieve successful treatment measures of formation damage.
- The source of formation damage should always be identified by the regular testing of the formation.
- Formation should not be allowed to reach a severely damaged stage before calling for stimulation because early stimulation would reduce treatment cost and the risk of treatment failure,
- Ensure that the formation to be treated is economically viable.
- Well test analysis should be carried out on newly completed well especially the explanatory wells to determine the onset of formation damage by indication like the skin factor and flow efficiency.
- Preventive measures should be carried out to prevent occurrence of damage than to allow it to occur. This is because; it is very difficult to remove damage completely from a well and also to save cost.
- Once formation damage has occurred, proper assessment and control practice will require the joint effort of geologists and engineers.
Hence I conclude that all wells are susceptible to formation damage to some degree ranging from minimum loss of productivity to complete plugging of pay zones. Te effective removal of formation damage can greatly increase the net present value of a well.
ACID STIMULATION PROCEDURE
The acidizing of Well X was carried out with the acid of Dowell Schlumberger coiled Tubing system as follows:
- All wellhead pressure were checked and recorded.
- On the long string, the surface control subsurface safety valve was tripped close for leak-off test and also on the long string, all the xmas tree valves were closed and confirmation was made to secure the long string.
- On the short string, the wire line lubricator was rigged up to test its closed in tubing pressure.
- Also on the short string, the surface control subsurface valve was retrieved. The stimulation was done with the aid of coiled tubing technology.
The coiled tubing activities carried out includes the following:
- i) The coiled tubing system was rigged up, then ran-in-hole with appropriate wash-tool and washed to 514ft while still circulating with diesel oil.
- ii) The string was prickled to clean out tubing as per treatment recipe attached. The essence of pickling is to remove rust/scale from coiled tubing (CT) and clean the produce tubing.
iii) The coiled tubing (CT) was pulled out of hole while circulating with diesel oil.
STIMULATION ACTIVITIES.
1) The various acids were mixed and pumped at various stages as stipulated by the pumping program.
2) After stimulation the well opened up and was produced clean.
3) The SCSSV was re-installed and actuated to open.
PUMPING SHEDULE TREATMENT RECIPE.
WELL X
- Pickle coiled tubing string
- Pump from diverter (3%NH4 CL+ F78 + Nitrogen)
- Pump stage 1
Pre-flush; 1500 gals 10% HCL pre-flush with:
6 gals A260 Corrosion Inhibitor
6 gals F78 Surfactant agent
30 gals U42 Sequestering agent
Main-Flush: 3000 gals 6.0% HCL/ 1.5% Hf (half strength regular mud acid) with
12 gals A260 corrosion Inhibitor
12 gals F78 surfactant
50 gals U42 Sequestering agent
- Pump from diverter (3% NH4CL + F78 + Nitrogen)
- Pump stage 2, repeat pre-flush and main-flush as per above
Pre-flush; 1000gals 10% HCL pre-flush with:
4 gals A260 Corrosion Inhibitor
4 gals F78 Surfactant agent
20 gals U42 Sequestering agent
Main-Flush: 2000 gals 6.0% HCL/ 1.5% HF (half strength regular mud acid) with:
8 gals A260 corrosion Inhibitor
8 gals F78 surfactant
40 gals U42 Sequestering agent
- Over flush with 300 gals Diesel + 300 gals U66
- Displacement with 30 bbls Diesel
REFERENCES
- T.Azizi, H.Chen& S.S Rahman Van Everdingen, A.F., ” The Skin effect and its influence on the prouctive capacity of a well”. Trans., AIME, Vol.198, pp.171,1953.
- Hurst, William, “Establishment of the skin effect and its impediment to fluid flow into a wellbore”, The Petroleum Engineers, October 1953.
- Roland F. Krueger., “An overview of formation Damage and Well Productivity in Oilfield Operations”. SPE-10029, JPT Feb. 1986
- Di Jioa M.M Sharma., “Formation Damage Due to Static and Dynamic Filtration of Water-Based Muds”. SPE-23823, SPE International Symposium on Formation Damage Control, Lafayette, Louisiana, 26-27 Feb. 1992
- Maly, G.P. “Minimizing Formation Damage-2. Attention to System Details Helps in Prevention of Damage”, Oil &Gas Journal (Mar. 29, 1976),pp 165-169
- Claus Marx, S.S. Rahman, “Evaluation of formation Damage caused by drilling fluid, especially in pressure-Reduced formation”. JPT, Nov. 1987
- Kenneth E. Porter., “An overview of formation Damage”., SPE-19894, JPT August 1989.
- R. Ghofrani, Y.Zhang and V.Boach., “New Method in Evaluating the formation Damage in Laboratory Investigations”. SPE-35151, internal Symposiumon Formation Damage Control, Lafayette, Louisiana, 14-15 Feb 1996
- Carmichael, R.S., “Handbook of physical properties of rocks”. Vol. III, CRC Press, Florida, USA, 1984
- Kaye, G.W.C, Laby, T.H.,”Tables of physical and chemical constants”. Langman, New York, 1973
