Summary

In recent years, with the constantly increased drilling number and deepening of the drilling depth, although the use of new technologies in rotary drilling method greatly improved drilling speed and quality, some of the problems that exist in the drilling process is cannot be ignored. This article is mainly focused on the lost circulation existed at this stage of the drilling technology project for briefly of analysis and summary. Address these issues put forward corresponding solution to better reduce the occurrence of lost circulation, ensuring the reasonable use of resource, reduce development costs, and promote the formation of oil can be fully exploited.

Introduction

lost circulation refers to a phenomenon occurred in the exploration and development of oil and gas drilling, Work over and other operations, the well working fluid (drilling, work over fluids, cement slurry, etc.) leak into the formation. The general lost circulation refers to a complex situation that the drilling fluid leaking into the formation during the oil and gas drilling process. Lost circulation will lose a large amount of drilling fluid resulting in drilling unsustainable and the drilling time will be some certain impact. When the lost circulation occurs, some necessary measures must be taken to plugging, resulting in material wastage situation and some other accidents will be easily caused such as well collapse, sticking, blowout, etc, which bring great losses for the drilling engineering.

There is a lot of necessity case for lost circulation, the voids, cracks or caves appear in the formations; fluids flow in the leakage channels can make a positive pressure existed between the wellbore and formation; in the formation must has enough space be able to accommodate a certain volume of drilling fluid, the lost circulation problems will be happen in the future when meet these conditions. In the event of lost circulation problems occurred in the future, must to judge the nature of the leakage layer and improve the success rate of well leak treatment. After the well leak occurred, the position of the leakage layer should be analyzed, after that analysis the lithology of the leakage layer, the depth and severity of leakage layer, and then propose the resolvent.

1 Project Overview

This article is taking a drilling engineering as a case, under normal circumstances, the upper part of the mine will be laying a layer of relatively loose sand, oil and gas are mainly distributed in the seven faults, five faults is east-south and other two faults is south-north orientation. The faults distance is between about 15 to 342 meters, and can be extend the length of about 3500 to 6000 meters, where the reservoir interstices of up to 20.4% and it is a high permeability layer. By the effects of its own surface and reservoirs, during the oil drilling process will be prone lost circulation. In recent years, with the ongoing development of water technology, had an impact on the reservoir rocks in the mine and lead to a gradual increase in pressure, the density of drilling mud has dramatically decreased, further the lost circulation happened repeatedly that caused serious economic losses.

Taking the mine depth of 1308 meters, 120 meters deep under the surface of the casing as an example, when the second drilling reaches 500 m depth, it will occur serious lost circulation and the leakage rate is 20 square meters per hour in continuous leakage, if it continues to carry out drilling, so that when a depth of over 800 meters will lead to the accumulation of mud, but this time the need for drilling cement prone to sticking phenomenon, which led to the use of loose blasting technology, reaming, casing seal blocking its drain layer, the whole process lasted about 40 days, the economic loss caused by the property up to more than 60 million yuan. In summary, to strengthen the anti-leakage of oil drilling is one of the main problems to be solved in oil drilling.

2. The reasons of lost circulation

Leakage phenomenon occurs in the drilling engineering from the subjective and objective two aspects analysis, the objective reasons include three aspects: one is the mine in the geographical environment is sand geology, to a certain extent, there is no hard rock, so prone to leakage phenomenon. In the second, the geographical location of the fault mine complex variety, prone to breakage can also cause an oil spill; thirdly, Reservoir permeability of soil is strong. The Subjective factors mainly refers to the process of oil drilling, the drilling construction personnel do not strictly in accordance with the appropriate drilling technology and requirements, not an accurate understanding of the exploitation of the petroleum geography environment, in the process of drilling most of them just with experience in drilling operation, which leads to a series of artificial lost circulation occurred during the petroleum drilling. In this regard, we must strengthen the drilling technology and the safe consciousness of anti leakage for the drilling personnel, and enhance their awareness of prevention, reduce some of the economic property loss caused by leakage.

3. leak proof measures

3.1 Technical Measures

(1)Properly control the drilling speed in the process of drilling, below each column should be controlled within 45~60s.

(2) If happen the well tower or sand bridge accidents during the drilling process which caused blocked, must be open the pump slowly and effectively reduce the displacement of the pump; adopt the appropriate speed to prevent the occurrence of pump suffocation, thus effectively reduce the lost circulation accident probability.

(3)During the drilling process, circulating mud should use the small displacement and during open the pump process should be paid attention to leakage section to avoid fix-point circle.

(4)The Kelly bar must be enhanced before the drilling pump put down completed.  Firstly the turntable to rotate more than a minute, then slowly start pump. Do not improve the pump rate before the pump pressure does not reach the stable value.

(5)In the loss occurred places, during the open pump process should be used small-displacement, and appropriate enhance the drill tool. After the returned drilling mud back to normal, using the drilling displacement circulation.

(6)Strengthen the supervision of the posts, each post personnel must be on the job, to be able to discover problems, and using the proper way to solve the problem.

(7)Must be fixed the control equipment installation, make the greatest possibility of reducing the solid phase contained in the drilling mud, reduce the annulus mud density, thereby reducing the pressure of the liquid column.

(8)In the process of drilling, must be grouting, in order to avoid the well tower and anti leakage layer to vomit.

(9) In the soft soil place construction, should take appropriate measures to prevent the bit balling

3.2Technical measures of mud in the construction process

(1) The viscosity of the slurry should be controlled by 40 ~ 60 s.

(2) Taking the pre-processing methods for the drilling mud, and also to fill in some of the new pulp, and thus the density decrease, solid phase decrease, the sticking and cutting reduced into low solid and high lubricity polymer mud, using this mud to drill those poor cementation upper gravel rock.

(3) In the suitable mud rock carrying case, adopt appropriate methods to reduce the slurry viscosity cut, so not only can reduce the annular pressure loss, but also reduce the pressure of excitement

(4) When drilling the high permeable geological, should reduce the amount of mud, and promote the quality of mud cake, to avoid the loop space  decreased due to the thick mud cake, and resulting the annuals pressure consumption to rise.

(5) In the process of weighted mud, should be uniform added weighting agent, avoid annular pressure loss due to the heavy mud rising too fast.

4. Plugging technology

Lost circulation is one of the common downhole problems in the process of drilling and well completion, in order to block the drain layer, must use a variety of plugging material, build a blockage in wellbore wall from close range leakage channel, used to partition the liquid leakage flow. Processing method of well leakage mainly include: analysis of well leakage causes, determine the leakage position, type and severity of occurrence of leakage; Lost circulation occurs in drilling, if conditions permit, as far as possible should be strongly drilling one section, to ensure that the leakage layer completely drilled, so as not to repeat the same problem of processing.

(1) If the leak rate of leakage less than 5 cubic meters / permeability hours, can stop the drilling, and to appropriate promotion, when do not have effectiveness to enhance the mud viscosity (60~70s), add 2%~3% of the DF-1, the present drill enters the drain layer still drill. If there is no effect, compound the plugging slurry again, then add 2%~3% of sawdust, and then injected into the floor necessary circumstances can use pressure way.

(2) If the leakage rate in the 5~20 cubic meters / hour, after the drill ascend to the safety position, compound bridging plugging slurry to 25~50 square metres, bridging agent types and dosage according to the well site to calculate leakage.

5. Conclusion

Generally speaking, good leak proof sealing should be taken in petroleum drilling engineering work, the author puts forward some points for attention:

 (1) Analyzing the causes of lost circulation accidents to determine the occurrence place, species and leakage condition of the leakage layer.

 (2) Scientific and reasonable design the construction of, construction process must be careful.

 (3) If occurs lost circulation in the drilling process, if conditions permit should be drilling in some dimensions, and then drilled drain layer, prevent the same problem is repeated treatment.

 (4) The specification of slurry must be achieved.

 (5) Then use the time pressure sealing, pressure should not exceed 3MPa, prevent leakage due to suppressing geologic strata.

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