Drilling and coring concept

Drilling and coring refers to mastering the underground geology.Directly obtain real and reliable information about underground rock formations, and use coring tools to remove large rock samples (cores) from the ground during drilling.

Conventional coring

Coring that does not have any special requirements for the core is called conventional coring.Conventional coring is the largest and most common in coring operations. No matter what oil and gas reservoir it is, a large number of conventional coring must be carried out in the exploration or development stage.

1. Main purpose
① Discover the oil and gas reservoir, understand the oil and gas content and reservoir characteristics, and determine the basic data such as the lithology, physical properties, thickness, and area of the oil and gas reservoir.

② Establish a stratigraphic profile and study the lithology, life and storage characteristics.

③Understand the lithology and electrical relationship.

2. Conventional coring method

①The general short-tube coring refers to conventional coring that does not take a single root in the middle of coring drilling.Its tool contains only one section of the core cylinder, and the structure is simple.It accounts for the largest proportion of the entire coring operation and can be carried out under any formation conditions.

②The coring of medium and long cylinders refers to the coring of a single root in the middle of drilling.Its tools must contain multi-section core cylinders. Usually, medium and long core coring is only carried out when the cementability and drillability of the stratigraphic rocks are good.The purpose of medium and long-tube coring is to increase the single-tube feed of coring as much as possible under the premise of ensuring a high core harvest rate, so as to greatly improve the core harvest rate and reduce the cost of coring.

3. Special coring

Drilling coring that has certain special requirements for the core is called special coring.It is mostly used in the oilfield development stage, usually in the following ways：

1) Oil-based drilling fluid coring refers to coring carried out under the conditions of oil-based drilling fluid.For sand and mudstone oil and gas reservoirs with large reserves, this kind of coring is generally carried out before development. The purpose is to obtain a core that is not polluted by the free water of the drilling fluid, in order to obtain more accurate data on the original oil saturation of the reservoir, which provides a basis for the rational formulation of oilfield development plans.Since the oil-based drilling fluid does not lose water, its performance is stable, and its fluidity and lubricity are very good, the core does not have the problem of water absorption, expansion or peeling, and it is not easy to break or wear. The core removed is regular, complete, good columnar formation, and high harvest rate.

2) Closed coring refers to sandstone oil fields mined by water injection. During the development process, in order to check the effect of water injection and development of oil fields, understand the washing situation of underground oil formations and oil-water dynamics, in order to formulate a reasonable development and adjustment plan, closed coring tools and closed fluids are used to remove the core that is almost not polluted by the free water of the drilling fluid under the conditions of water-based drilling fluid.Due to the high cost of oil-based drilling fluid coring, closed coring can replace oil-based drilling fluid coring under the premise that the quality index of closed coring is guaranteed.

3) Sponge coring refers to the coring of the inner core cylinder with a special sponge liner.Using a pre-saturated sponge liner, a core with a fairly accurate oil saturation can be obtained under the conditions of water-based drilling fluid.This is a kind of centering developed abroad in recent years. The process structure is not too complicated, but the cost is high, and it is suitable for medium-hard to hard strata.

4) Pressure-holding closed coring refers to the late stage of the development of sandstone oil fields. In order to accurately obtain information such as reservoir fluid saturation, reservoir pressure, relative humidity and reservoir conditions under the bottom conditions at that time, in order to formulate a reasonable development and adjustment plan to improve the final recovery rate of the oilfield, pressure-holding closed coring tools and closed fluids are used. Under the conditions of water-based drilling fluid, the core that maintains the integrity of the reservoir fluid is drilled.That is, drilling a core that is not polluted by the free water of the drilling fluid and maintains the pressure of the reservoir under the conditions of the bottom of the well at that time.This kind of coring equipment and technology is more complex and costly, and it is suitable for soft, medium and hard formations with diagenesis.

5) Conformal coring of loose sandstone refers to the coring that maintains the original (pre-cylinder) shape of the core in the loose sandstone formation.Because in loose sandstone formations, due to the low strength of the core and the failure to form a column, the core often forms a pile of loose sand after it exits the cylinder, and the physical properties of the core cannot be obtained.Therefore, maintaining the original shape of the core and avoiding man-made damage has become the technical key to conformal coring.At present, multi-stage double-flap combined core cylinder, rubber cylinder, glass fiber reinforced plastic inner cylinder and composite lining cylinder can all meet the requirements of conformal coring, but only multi-stage double-flap combined core cylinder has low cost and easy to use.

6) Directional coring refers to the process of exploration and development of oil and gas reservoirs, in order to intuitively understand the tectonic parameters of the reservoir, comprehensively grasp the complexity and changes of the geological structure, formulate an economical and reasonable exploration and development plan, and adopt directional coring technology to take out the core that can reflect the tectonic parameters such as formation inclination, inclination, and direction.Directional coring is only suitable for strata with good core columnar formation.

Coring tool

Composition of coring tools

The coring tool consists of an upper stabilizer, a water separation joint, a hole-blocking steel ball, an outer return nozzle, a suspension assembly, an inner core cylinder group, an outer core cylinder group, a lower stabilizer, a combined inner cylinder shoe, a core claw, a coring drill bit and other components.

The main function of the coring drill bit is to form the core, while the main function of the coring tool is to protect the core and remove the core.Obviously, it is impossible to take the core of the rock with high quality without a suitable coring tool.According to the coring method, the coring tools at home and abroad can be basically divided into three categories: pressurized, self-locking and sand card type.    

When cutting the core, the drilling tool is lowered, and the core claw shrinks and deform by the drilling pressure and the sliding impact force of the inner cylinder. The coring tool that wraps, clamps or twists the core is called a pressurized coring tool.When the drilling tool is lifted up when the core is cut, the core grasps its own elastic force or the friction force between it and the core shrinks and deform, and the coring tool that clamps and pulls out the core is called a self-locking coring tool.

Coring tool selection

1. Selection of coring tools under different well depth conditions    

Choose short tubes for shallow wells and medium and long tubes for deep wells.

2. Choose the centering tool according to the stratigraphic lithology   

Pressurized coring tools should be used in loose and soft formations, while self-locking coring tools should be used in medium-hard ~hard formations and soft formations with good core columnar formation.

At present, diamond coring drill bits are widely used in drilling coring at home and abroad.

1) The diamond coring drill bit has a good effect, especially the tire-type diamond coring drill bit, which can ensure that the edge of the drill bit is uniform.The diamond is wear-resistant and the carcass is resistant to erosion, so the drilling is stable, the speed is fast, the harvest rate is high, the service life is long, and the comprehensive economic benefits are good.

2) Diamond coring drill bits have a wide range of applications.From extremely soft to extremely hard formations, there are various series of diamond coring drill bits suitable for them to choose from, which can fully meet the needs of coring under various conditions.

3) Carcass diamond coring drill bits are easy to form, easy to process, and high yield.

Preparation before taking the heart

Borehole preparation

1) It is necessary to maintain good well body quality at the beginning of drilling to prevent well slope, dogleg, keyway, shoulder, diameter reduction, etc.

2) A drill bit before coring should be equipped with a salvage cup, and the bottom of the well should be leveled.Large-displacement circulation removes sand from the bottom of the well, and short-term drilling is carried out if necessary to ensure that the bottom of the well is clean, smooth, and smooth.

3) Continuous coring, expand your eyes every 20 meters you drill into.

Preparation of equipment, instruments, and drilling tools

1) The equipment and instruments should be inspected and maintained before coring, to ensure that drilling does not stop during coring.

2) Conduct a comprehensive inspection of the drilling tools to ensure that there are no thorns, leaks, or buckles during coring.

3) Further inspection of coring tools, accessories, auxiliary tools, and drill bits should be carried out to avoid downtime due to tool accessories.

Coring operation procedure

1．Coring tool assembly

1) Flatten the coring tool on the ground, use chain pliers to tighten the connecting threads of each section, and wear wire guards at both ends.

2) Check whether the outer diameter of the stabilizer matches the drill bit, and measure the length of the coring cylinder.

3) After hoisting the rig, use the crane card to go down into the well, use the kava card under the upper stabilizer, and use the safety kava card.

4) Remove the safety joint, raise the inner cylinder, connect the inner cylinder shoe with the core claw to the inner cylinder, and tighten it with chain pliers.

5) Go down into the inner cylinder, check whether the safety joints and bearings are flexible, apply threaded oil (not too much), use chain pliers to tighten the buckle, and finally use large pliers to tighten the buckle.

6) Lift the core cylinder out of the wellhead, remove the lower guard wire, and use a special ruler to check the longitudinal gap, which is generally 8~13 mm.If it is not suitable, it should be adjusted by adding or subtracting the gasket, and then connect the checked coring drill bit, and the connecting thread of each section must be tightened with pliers.

2．Drill down

For the well section with reduced diameter, it should be entered slowly. If it encounters resistance, the pump circulation should be opened first, and the short well section should be carried out with low drilling pressure, low speed, and large displacement. Drilling requires stable operation, and must not be violently released, slammed, braked, or turned violently to prevent the drilling tool from swinging violently.Draw your eyes in the well, obliquely, and for a short time.It is forbidden to use a coring drill bit to make large-scale (more than 10 meters) eye-catching or forcibly drill down.

3．cycle

When drilling down from 0.5 to 1 meter from the bottom of the well, circulate the drilling fluid, move smoothly and rotate the drilling tools appropriately.Correct the finger weight meter, probe to the bottom, lift the drill tool and unload it, put it into the steel ball, and wait for the pump pressure to rise, and do the drill bit pressure drop test.

4．Heart-making

First start the turntable, slowly apply the drilling pressure, and test the operation at low speed and light drilling pressure. After the operation is stable and the drilling is 0.3~0.5 meters, it will gradually be adjusted to the normal coring drilling parameters.

5．Get in

 (1) Drilling pressure

The drilling pressure during coring drilling should be determined according to the hardness and drillability of the rock, the type of drill bit, the structure and strength of the drilling tool.The drilling pressure must be greater than the compressive strength of the formation, otherwise, there will be no ruler.If the drilling pressure is too small, the teeth of the drill bit will eat less into the formation. Although there is a ruler, the drilling speed is very slow, and the drill bit will wear out quickly; the greater the drilling pressure, the deeper the drill bit will eat into the formation; the higher the drilling speed.

The coring of diamond drill bits currently used, the cutting effect mainly depends on the degree of stable contact between the dry diamond and the drilled formation, the speed remains the same, the drilling pressure is increased, and the change in drilling speed is observed. If the drilling speed no longer increases or the increase is quite slow, or the torque increases greatly, then the drilling pressure at this time is the optimal drilling pressure.If a more satisfactory drilling speed has been achieved, the drilling pressure should be kept unchanged and the drilling should be evenly delivered.

(2) Speed

 In theory, under certain limits, if the depth of the drill bit into the formation remains the same and the rotation speed increases, the drilling speed will also increase, and the time for the core to enter the cylinder will be shortened, which is conducive to improving the core harvest rate.However, a higher speed will cause the drill bit to swing, increasing its lateral force and the centrifugal force of the drill string, which is detrimental to the protection of the core and the drill bit.In fact, the work of the drill bit at the bottom of the well is also affected by many other factors.Therefore, in order to protect the core, a low speed should be used to reduce the centrifugal force of the core cylinder.

(3) Displacement

The size of the displacement is appropriately selected on the premise that it can meet the requirements of cleaning the bottom of the well and cooling the drill bit.The displacement is too large, and the drilling fluid coming out of the water hole of the drill bit has a large impact force. Although it does not directly hit all the cores, it is also a destructive force to the roots of the core for soft formations or formations that are not hard enough.In order to protect the core, the displacement should be smaller.

6．Take a single root

In the process of in-progress and long-tube coring, a single operation must be carried out.The method of receiving a single root of the self-locking coring tool is first to stop drilling, stop the pump, and then lift the drilling tool until the core is cut.If the lifting tension reaches a certain value and the core is still cutting, the pump can be kept circulating under this tension until the core is cut.After the core is cut, keep the drilling tool from rotating, lift it smoothly for 2 to 3 meters, and then slowly lower it to 0.3 meters from the bottom of the well, observe the changes in the barometer, and confirm that there is no core remaining at the bottom of the well, you can take a single root.After receiving the single root, go down into the drilling tool, apply a drilling pressure 50% larger than the coring drilling pressure to loosen the core claw, lift the drilling tool to restore the original suspension weight, and drill according to the method of coring at the beginning.

7．Cut heart

 (1) Heart-cutting under normal circumstances

When coring medium-hard or hard formations with uniform lithology and good columnar formation, the position of the core cutting can be determined according to the length of the inner core cylinder.When coring in strata with uneven lithology and complex changes in soft and hard interlacing, try to choose the coring position in the harder strata within the range of the length of the inner core cylinder, and don't force more feet.Cutting the core is like cutting the core before receiving a single root. After cutting the core, the drilling tools should be raised at different heights, and the drilling tools should be slowly lowered in different directions to explore the core.If there are more spare hearts at the bottom of the well, in order to obtain all the accurate information, a “set of hearts” should be carried out.

(2) Heart-cutting under abnormal circumstances

During coring drilling, if the drill bit mud pack is found, the circulating waterway of the drill bit is blocked, and abnormal phenomena such as “grinding heart”, “stuck heart”, “blocked heart”, and “heartburn” cannot be ruled out due to abnormal coring tools or stratigraphic factors, the heart must be cut immediately.

8．Start drilling

The drilling operation should be smooth, and the drilling tools should be lightly lifted, lightly lowered, and slowly unloaded to prevent the drilling tools from swinging violently, and it is forbidden to use the turntable to shackle.

9．Core out of the cylinder

Whether on the drilling platform or in the site through the core, strict attention must be paid to the sequence of the core out of the cylinder to prevent the core from misalignment.The method of operation is to first remove the safety joint, lift up the inner tube, rinse it clean, use chain pliers to loosen the inner tube shoes 2 to 3 buckles, pull the inner tube to a place where the driller can easily see, tie the inner tube with a rope, remove the inner tube shoes, install the core pliers, and then the driller operates, and releases the core as soon as it is up.

If it is aluminum alloy and glass fiber reinforced plastic pipe coring, after removing the safety joint, lift it up by 0.2 meters, use a chuck to jam the inner core cylinder, and sit on the outer cylinder, remove the safety joint with chain pliers, use a lifting guard wire to lift the inner cylinder and core off the drilling platform, place it flat on the site, rinse it with water, and cut it in segments with a chainsaw according to geological requirements, seal it with a core cap, and clamp it with a clamp.