Fracturing except in special circumstances, the construction process is the same, can be roughly divided into seven steps.
Circulation, pressure test, Trying to squeeze, Fracturing, sandfrac, For extrusion, back wash
The above seven processes of hydraulic Fracturing of district and the first line, generally, have detailed measures and records in each process. And for the more familiar with the field, the above steps is not too obvious. Especially there is no obvious boundaries between Trying to squeeze and Fracturing process, but according to the time and pressure to determine empirically by sandfrac process.
1.circulation
The circulation pathway is circulation fluid from tank fracturing blender truck Pump, all Fracturing cars, and then by Fracturing circulation Pup joint along the pump function returns tank. Circulation is a single by car, gear is gradually increased. Circulation fluid is set according to the site conditions, can be used in water, oil or fracturing fluid.
The purpose of circulation is to identify all kinds of performance equipment, check the ground circulation Pup joint is open, all kinds of pump water supply situation is good. At the same time through the circulation can make the tank fracturing fluid mixing, in order to achieve the temperature and viscosity of the uniform, winter construction can also remove dead oil in cold weather due to formation of block tank export phenomenon.
Circulation standard is based on the circulation outlet discharge normal qualified. In the winter construction can prolong circulation time according to the specific situation.
2. Pup joint pressure test
Pup joint test pressure should be wellhead the total gate locked, with water, oil or fracturing fluid pressure test.
The pressure test is to test the device, wellhead, ground and all threads, the connection part of the Hammer union is able to withstand the pressure effect. By the pressure test inspection, we can ensure that the work does not appear in the Hammer thread, and union was suppressed leakage or explosion phenomenon.
Pressure test, a general test pressure burst pressure prediction of 1.2 ~ 1.5 times. Keep 2~3 minutes buck for qualified pressure rise.
Pressure test pressure of Fracturing pump, slowly, to prevent the pressure test equipment, the ground of Pup joint and wellhead as a result of accidents caused by insufficient strength and fixation.
3. Trying to squeeze
Ground pressure test, open the gate, then you can start one or two Fracturing pump the wellhead filled with fracturing fluid, and then gradually increase the pressure to Trying to squeeze, the formation pressure is stable, until now.
Trying to squeeze, is to check the well of string components (such as packer, sandblasting, hydraulic anchor and so on) is working properly, check the string well into the position is correct, it is more important to the reservoir through the Trying to squeeze can control the water absorption index strata, in order to understand and master the characteristics and reservoir estimation of maximum burst pressure.
Measurement of formation water index, should be controlled below the formation of fracture displacement required displacement, every 2 to 3 minutes from time to time to record a pressure and displacement, generally need to measure 2 to 3 ideas, in order to obtain the average water absorption index.
Water absorption index represents the ability of crack formation before the absorption liquid, it means: well every day a number of atmospheric pressure difference can absorb liquid.
Water absorption index only in Nii or district, it is necessary to detailed calculation of formation fracture pressure grasp unclear circumstances, to understand and grasp the rules of Fracturing and the area, without the need for detailed calculation.
4.Fracturing
In the Trying to squeeze stress and displacement after a stable, if the water absorption index to measure have been determined, then by car to start or start at the same time a few cars or vehicles, in a very fast speed to the Pump fracturing fluid in the wellbore, the pressure rises rapidly, until it is equal to or more than formation fracture pressure demand so far, the formation of cracks.
Fracturing is a process of intense but orderly, to the expected Fracturing results obtained with as little as possible fracturing fluid. The judge whether cracks formation should be timely, accurate. It is one of the very important and key.
How to judge whether the formation of cracks? Field personnel worked out a lot of experience in the long-term production practice, here only to several methods are described below.
1 according to the change of stress and displacement of the judgment whether the formation of cracks
In the Fracturing, as the pressure is increased with the increase of displacement, keep the ratio relationship between them. But when the formation of cracks, the original ratio relationship between them was broken and there are two kinds of case: the first kind is that once the formation of cracks, the pump pressure decreased rapidly rising displacement. This situation is more, is the most easy to grasp the command staff; second is a constant pressure, and displacement increase. This phenomenon is often encountered in the field, its reason, at the scene of some wells in the formation of cracks in the formation is not a process of mutation, but the original crack extension and expansion, it has little effect on the pump pressure, some even only a few large pressure drop well, so that small changes in pressure, the pump pressure on the pressure table fluctuation and the effect of liquid friction is also hard to see, for example, the formation of cracks in the ground, the bottom hole pressure drop, but because of increased resistance to friction and displacement increases, the results from the ground pump pressure on the table: a downward trend, even there is slightly increased.
For this phenomenon, should be increased to a larger displacement that is the formation of cracks. For example, some wells in the Trying to squeeze when the pressure is 150 ATM, the displacement of 0.3 m 3 / min, Fracturing pressure up to 300 atmospheric pressure, and the displacement of 1.3 m 3 / min, at this time can be considered the formation fracture has been formed.
2 according to the change of mechanical equipment is formed to determine crack
For the formation of cracks, can also be seen in the changes of the Fracturing reaction on mechanical equipment. Before the crack has not been formed, because the pressure is high, the load of the diesel engine on Fracturing, the sound is very heavy, at this time, a great flame or smoke. Fracturing blender truck from the sand tank can be observed, the displacement is not a liquid, liquid sand tank is also small. But dawn crack formation, whether or fracturing blender truck diesel engine are changed, the diesel engine will change the sound, a sudden increase in the liquid displacement of sand tank, a great churning waves, especially the control of pump outlet valve must be open in order to make the liquid supply to.
Judging by this method the crack formation, must have the rich experience to master the changing standards, in order to determine the accurate and timely.
3 use of formation water absorption index to judge the crack formation
The formation of cracks will lead to changes in formation water absorption index, which is no doubt. The formation of cracks is more reliable, accurate judgment, science and the use of this method.
When the formation is not the formation of cracks, the absorption area and permeability formation itself is fixed, then the injectivity index is a fixed value, the absorption capacity and the formation pressure growth will also be a fixed value, but when the formation of cracks, due to the crack area, so that the formation absorption area increased, thus breaking the proportional pressure and displacement of the original, so that the ratio is greatly increased, and resulted in a difference.
Water absorption index Fracturing is significantly more than the water absorption index strata, it has formed a crack formation.
Multiple water absorption index growth is that the crack extension, the deeper, increased water absorption index increased with the Fracturing of the ground pressure of the pump displacement increases.
5. sandfrac
The local layer has been broken down, can immediately after the stable pressure and flow rate of sandfrac. Sandfrac is an important step in the Fracturing process, you should quickly check the pump is working properly, Pup joint, wellhead devices, if there are problems, can withstand the pressure shock, if not sure or leakage phenomenon, should be quickly stop to repair, so as not to affect the quality of construction.
In the process of sandfrac, we should start from small to large, sandfrac sand mixing ratio can be controlled in 5 ~ 7%, at this time can be verified whether to crack formation. According to the Fracturing layer depth estimate the wellbore and ground tube volume exchange, according to the displacement, we can know the Sand carrying fluid how much time into the ground, when the sand has entered into the cracks, stress and displacement are relatively stable, should improve the sand mixing ratio.
If the formation is not the formation of cracks, the sand can not enter the formation, this will lead to increased pressure phenomenon. If there is such a phenomenon, should immediately stop the pump of back wash, the wellbore sand all the back wash, so as not to deposit in the wellbore.
When the normal sandfrac, general mixed sand ratio from 15 to 20%, with high viscosity fracturing fluid sand mixing ratio referred to 40 ~ 50%.
In the process of sandfrac pump pressure will have two kinds of display. The first is with sand ratio increasing. The pressure decreased gradually. The causes of this phenomenon is that with the wellbore increases with the sand ratio, fracturing and the ratio of fluid increased, the effect on the formation pressure is constant, the ground pump pressure will be reduced and fall. The second is displayed in the sandfrac process, the pressure will be on a small range of swing, swing range in general between 20 ~ 30 atm. This phenomenon except for plunger pump effects caused by the impact, but the main reason is the deposition process in the cracks in the sand is a dynamic equilibrium process. When the Sand carrying fluid into the cracks, there will be a part of the sand began to sink, with the accumulation of sand, the crack width is smaller. So, in the displacement under the condition of constant Sand carrying fluid increase in the crack at the entrance velocity, reflect a small rise in the pump pressure on the pressure. When the liquid flow rate increased, and the deposition of sand pile in advance to make the width of crack again to expand the liquid flow rate decreased, the ground pump pressure and there will be a small drop. Such a dynamic balance process caused by the pump pressure swing is a normal phenomenon, it should be distinguished from the pressure oscillation caused by sandfrac is not normal.
In order to ensure the normal construction, even in the process of sandfrac sandfrac. To prevent the formation of sand, sand plug bridge, at the same time in the process of sandfrac Fracturing pump should be maintained in good working condition, otherwise it will decline because of stress, displacement and crack closure phenomenon caused by sand plug.
In the Fracturing and sandfrac process, Fracturing pump often appear to the phenomenon, this phenomenon not only when the pressure is big swing, and make the pressure, displacement decreased, especially in the sandfrac process, this situation is very dangerous.
The main reason is to produce fracturing blender truck liquid supply shortage or Unicom Pup joint blocked or air into the pump. Appear this kind of circumstance should first determine the cause of this phenomenon, and then take effective measures.
6. For extrusion
After a predetermined amount of sandfrac added, can immediately replace Pump solution, in order to sand carrying fluid displacement to fracture. Replace the liquid quality is very important in the process of For extrusion, is directly related to the success or failure and the effect of Fracturing. Displacement fluid is too much, will put the sand into the depths of the crack near the wellbore, pressure drop, so that the crack closing near wellbore, not up to expectations, the effect of the Fracturing. Displacement fluid is too small, can make a part of more than sand deposited slowly in the wellbore formation sand in accident.
Sandfrac after the cessation of wellbore liquid proportion gradually decreased, when the pump pressure will increase. In order to make the crack opening state when the holding time is sandfrac, the For extrusion should be appropriate to increase the throttle, effect of increasing displacement of pump pressure rise due to the reduction in emissions.
7.Back wash or string
For extrusion back can be taken immediately after wash or string measures according to the construction situation.
Fracturing technology is currently the vast majority of stratified based on packer pressure selection process. Therefore in the construction is finished, there will be a small part of the waste sand remaining in the wellbore and packer card from the inside, this part of the residual sand although is small, but sometimes cause sand in accidents, this process is essential. The above six processes, if the construction goes well, as long as the activities of string, packer can accelerate the recovery of the construction; if not successful, it will have a lot of wellbore sand effect of packer recovery, at this time should be back wash, the card from the back wash and the sand cleaning shaft, to ensure packer smooth recovery, prevent sand in.
The above seven processes can be shut after removing Pup joint, then Fracturing is complete, according to the measures for the replacement of string well production. Must be stressed that special attention to prevent any measures taken to reduce the discharge or a sudden pressure to Fracturing, to avoid Fracturing sand is ejected from the crack, the effect of the Fracturing.