Complex downhole conditions and treatment measures

1. What is downhole complexity? What is a drilling accident?

A: In the drilling process, due to the special formation, improper drilling fluid type and performance, poor well quality and other reasons.

The phenomenon that can not maintain normal drilling and other operations. Such as resistance jam, serious hop during drilling, loss, kick, etc., are called downhole complex conditions.

Due to operational errors and improper measures to deal with complex downhole conditions, such as drilling tool break, stop drilling, stuck drilling, blowout and other drilling accidents will be causing.

2. What are the causes of the loss? What kind of formation does the loss occur in and what phenomenon does the loss occur in?

A: During the drilling process, the leakage of drilling fluid or cement slurry into the formation is calling a loss. Loss is one of the serious and common downhole complex conditions in drilling.

Loss occurs when the pressure of the drilling fluid column, in the well is greater than the formation rupture pressure during drilling.

There are two main factors that cause well loss: one is abnormally low fracture pressure of underground formation, large porosity and good permeability of rock strata, or there are cracks and caves, etc. On the other hand, due to improper drilling process measures, such as drilling fluid density is too high, the pressure is too high, or the pump is too strong and the drilling speed is too fast, resulting in downhole pressure excitement.

Loss can occur in one of four formations:

1. Loose or high permeability strata;
2. Natural fractured strata;
3. induced fractured formation;
4. Caving strata.

Downhole.The loss will cause the drilling fluid pool surface to drop and the amount of drilling fluid returned will decrease.

Severe loss will make the drilling fluid lose circulation and can only enter and leave, which will lead to the collapse of the borehole wall or a blowout.

3. According to the properties of the reservoir, the rate of loss and the cause of the loss, the type of the loss is described briefly.

A: The amount of drilling fluid lost per hour is calling the loss rate, which can be divided into three main categories basing on the properties of the loss zone, the loss rate, and the cause of the loss.

1) Leakage of permeability. This type of loss occurs in shallow well intervals in loosely consolidated sand and conglomerate formations. Due to the good permeability of these formations, the drilling fluid will leak into the space of the rock layer under the action of the pressure difference in the well, but the formation of mud cake will prevent or weaken the extent of the leakage, so the leakage rate of the permeability loss is not large, generally within 10m3/h.

2) Fracture leakage. Natural fractures may exist in various types of rock strata encountered in drilling, and drilling fluid loss will occur in different degrees when drilling in strata with natural fractures. When drilling in the fractured zone formation, loss often occurs due to the phenomenon of downhole La and accelerated ROP, with the loss rate ranging from 20 to 100m3/h.

3) cavern leakage. In some limestone formations, large karst caves have been formed by the long-term dissolution of groundwater. When drilling into a large cave, there will be a hole in the drilling tool, sometimes up to 4 ~ 5m, and then circulation failure, drilling fluid can only enter. Generally, the leakage rate is more than 100m3/h. Blowout or well collapse and sticking accidents often occur after the loss, which is the most serious loss.

4. What are the commonly used plugging methods and plugging agents?

A: 1) Treatment of permeable losses. Slight loss can be achieving by adjusting drilling fluid properties. Reducing downhole pressure difference and blocking the rock void passage to achieve normal drilling. The density of drilling fluid can be reducing, balancing drilling technology can be adopting. And the viscosity and shear force of drilling fluid can be appropriately improving to increase the resistance of drilling fluid flowing into formation void. The commonly used treatment agents to improve viscosity and cutting force are CMC. Baking glue, soda ash, sodium silicate, glauber’s glass, etc. If the treatment effect is not obvious after improving the viscosity.Fine particle bridging agent such as fine mica sheet and nut shell powder can be adding, to the drilling fluid for circulation until plugging.

2) Treatment of fracture leakage. Due to the different specific conditions of fracture leakage, the leakage rate varies greatly. In the specific treatment, the appropriate method should be selecting according to the specific situation. At present, the following three methods are commonly using in China:

① Filling husk, sawdust thick drilling fluid plugging. Grain husk and sawdust should be fine, the addition amount is 10%~15%, and the viscosity should be appropriately high. Because sawdust and grain husk are easy to block the drill hole, reverse circulation injection can be using.

(2) When fracture leakage occurs in non-oil reservoir, cement or glial cement can be injecting to plug the leak. One formulation of colloidal cement is: 5 parts cement, 1 part clay, plus 2% calcium chloride as a coagulant. At present, there is a lot of cement plugging in the field. The key of cement plugging is to master the position of cement slurry in the well, so that at initial setting, most of the cement slurry enters the formation and a small amount of cement plug is left.

(3) When fracturing leakage occurs in the oil and gas reservoir. The fracturing oil and gas reservoir can be temporarily blocked by limestone injection drilling fluid. The formula is the mixture of lime milk (density 1.3g/cm3) and drilling fluid (density 1.5g /cm3 or above), and the ratio is 1:1 or 1:2. The specific time should be determining according to the downhole temperature construction requirements. The other is quick-setting lime milk drilling fluid. Which is preparing by adding water glass to ordinary lime milk drilling fluid. The content of water glass can adjust the initial setting time. The larger the content of water glass, the shorter the initial setting time.

In order to prevent the keyway sticking, it is necessary to ensure the well quality and avoid the dogleg section. During the drilling trip or the next drilling trip. The hole should be repeatedly drawn in the keyway well section to destroy the formation of the keyway in time. When the drilling trip reaches the keyway well section. The hole should be starting slowly at a low speed, and high-speed lifting is strictly prohibiting.

5. How many ways to deal with stuck drill? Which types of stuck drill are they applicable to?

A: After the stuck drilling accident occurs. First of all, it is necessary to analyze the situation of lifting, running, rotating and opening the pump. As well as the hole situation and various phenomena before the stuck drilling. Accurately determine the cause of the stuck drilling, and then take corresponding measures. But no matter what kind of the stuck drilling nature. It is necessary to pay attention to adjusting the performance of drilling fluid, timely removal of cuttings, cleaning the hole. The following methods are commonly using to uncard.

1) Lift up, (lower down) and rotate the drilling tool to unstick;

2) Bath well dissociation card;

3) Up hit, (down hit) uncard;

4) Reverse sleeve milling and solving card;

5) Explosion reverse, sleeve milling;

6) Explosion and lateral drilling of new boreholes.

Sticky suction stuck drilling, collapse stuck drilling, sand bridge stuck drilling, reducing diameter stuck drilling, keyway stuck drilling, mud bag stuck drilling, dry stuck drilling can be using in the above method from top to bottom until the accident is relieving.

6. What are the characteristics and steps of the reverse sleeve milling and unloading card and the explosive reverse sleeve milling and unloading card?

Answer: downhole

1) the reverse sleeve milling solution card is a more complex processing method, time-consuming. The steps of reverse buckle and sleeve milling are as follows:

(1) Lift all the weight of the drilling tool above the stuck point (and slightly larger).

So that the drilling tool near the stuck point is not subject to tension and pressure. And then reverse the rotary table. Reverse the drill pipe wire buckle, and pull out of the well.

(2) If the pour out of the drilling tool is less than the card point. Can be using under the reverse buckle male cone (if necessary in the male cone above the safety joint), down to the drilling tool in the well to make the buckle, and then up. So that the stuck point of the drilling tool is not stressing, and then reverse the turntable reverse buckle. So repeated until the card point all the drilling tool out, then sleeve milling.

The outer diameter of the outer diameter of the outer diameter of the outer diameter of the outer diameter of the outer diameter of the outer diameter of the tube is slightly smaller than that of the hole. For example, the outer diameter of the outer diameter of the outer diameter of the joint of the drill pipe can be 165mm(6-5/6in). When the drilling pipe of 119mm(4-1/2in) is milling.

The length of the casing milling barrel is generally about 50m, or even longer, which is connecting under the drill pipe to the well. After the drilling tool in the well is casing, the casing milling is carriing out. After the length of a casing milling barrel is casing, the drilling is pulling up, and then the reverse locking drill pipe and the reverse locking male cone are lowering for reverse locking. So repeating (casing milling – reverse locking – casing milling) is carriing out until all the drilling tools in the well are emptiing out.

2) Milling and unsticking of explosive reverse sleeve is a new reverse method to deal with stuck drill accidents.

Firstly, the location of the stuck point is measuring. And then the detonating cable is sent from the drill hole to the first joint wire buckle above the stuck point. When the middle of the detonating cable is aligning with the joint, all the weight above the stuck point is lifting. And a certain reverse moment is appliing to the drilling tool to ignite the detonating cable to make it explode. When detonating the detonating cable, violent impact and powerful seismic force are generating, which is enough to cause elastic deformation of the joint part and timely reverse the buckle.

This is the same principle as that when the drill pipe joint cannot be discharging. It can be discharging after hitting the drill pipe female joint with a sledgehammer.

At the same time, the explosion of the detonating cable generates a lot of heat.

Which heats the drill pipe joint, melts the wire oil in it, and produces plastic deformation, which also helps to release the wire.

This method has the advantages of safety, reliability, fast speed, the drilling tool is not easy to destroy.Do not need the reverse buckling drilling tools and fishing tools, and accelerate the speed of processing stuck drill, but to strictly control the amount of explosives. And reasonable operation, reverse milling sleeve, fishing.