Fracturing acidification is referred to as acid fracturing. It is fracturing without proppant by using acid as a fracturing fluid at a higher pressure than the formation fracture pressure. During acid fracturing, the wall of the fracture is dissolved into an uneven surface by the dissolution of the acid liquid, so that the wall of the fracture will not be completely closed after the pump is stopped and the pressure is relieved. Therefore, it has a high conductivity and has obvious effect on restoring and improving the capacity of the oil well. This method is suitable for carbonate rock reservoirs.
Technical introduction
The acid treatment process of acid extrusion in the formation under the pressure sufficient to crush the formation to form cracks or open the original fractures of the formation is called fracturing and acidification. It can be divided into pre-liquid acid fracturing and ordinary acid fracturing (or general acid fracturing).
Fracturing acidification is mainly used to plug oil and gas wells in deep or low permeability areas. Fracturing acidification where the acid injection pressure is higher than the rupture pressure of the oil (gas) layer is customary to call acid fracturing. Acidizing hydraulic is a widely used measure to increase production and injection in limestone reservoirs at home and abroad. It has begun to become an important completion method.
The acidification fracturing process is mainly limited to carbonate reservoirs, limestone and dolomite, and has two purposes: ① to relieve formation damage; ② to transform undamaged reservoirs.
Since the introduction of sand fracturing in carbonate reservoirs (initially confined to sandstone reservoirs), acid fracturing has gradually replaced sand fracturing, but this substitution is generally ignored. This is understandable because the essence and process of the two processes are fundamentally similar, i.e. the formation of a long fracture channel with some conductivity extending from the wellbore to the reservoir.
In both cases, the height of the fracture is mainly controlled by the difference in stress in the boundary layer, and the length of the fracture is mainly determined by the height of the fracture and the filtration of the fracturing fluid. Sand fracturing is preferred for the following reasons, namely that it is easy to simulate (no reactive fluid; filtration is relatively stable), and by definition, it does not use the acid that is still feared or even avoided. At the same time, under the same reservoir and the same construction process, only the amount and type of acid used previously are considered.
The conductivity of long-term acid-etched fractures produced by sand fracturing and acid fracturing is compared, and it is found that sand fracturing is more advantageous.
In acidification fracturing, acid is injected into a crack created by a viscous fluid (pre-fluid), or the acid itself is used to form the crack. As the acid flows along the crack, it dissolves the fracture surface. If it is uneven (differential) dissolution, it retains a certain conductivity when the crack is closed.
Remarks: Pickling - no external force or slight agitation, for sandstone, carbonate rock skin plugging or perforation hole clear, wellbore scaling and thread oil clear.
Construction technology
There are two basic methods for obtaining acid-etched fractures in carbonate reservoirs, which remain the foundation of the acid fracturing process:
Viscous fingering
(Pre-fluid) Viscosity refers to the advance (pre-fluid): The formation is first pressed open with a non-reactive, highly viscous cross-linked gel solution to create the desired fracture geometry (e.g. length, height, and width) and to cool the reservoir to slow down the reaction with the reservoir after acid injection. After injecting a non-reactive pre-fluid, a less active acid (HCI or a mixture of HCI and organic acids) is pumped into the formed fracture.
Thickening acid fracturing
Thickening acid fracturing operation design includes: pre-liquid, thickening acid stage, acidification replacement liquid.
The precursor fluid can form cracks and reduce the temperature around the cracks, and is a typical thickening water. The purpose of the thickening acid stage is to widen the crack and etch the crack non-uniformly. This stage usually uses thickening acid, emulsifying acid or foaming acid (or their mixtures).
In all carbonate rock acidification operations, hydrochloric acid with a mass fraction of 15% is commonly used, but sometimes high concentrations of hydrochloric acid, organic acids, and mixtures of hydrochloric acid and organic acids are used. Many acid fracturing operations use thickening acids, and xanthan gum is a good thickener for 15% hydrochloric acid; the only drawback of xanthan gum is that it is difficult to degrade below 93 ° C, but when the concentration of hydrochloric acid greatly exceeds 15%, it will degrade very rapidly.
The thickening acid polymers in use today are a variety of different polypropylene phthalamine thickeners. Polypropylene phthalamines can be used at both low and high temperatures, and they can also be cross-linked to achieve high viscosity and gel stability. When the acid is diluted or consumed (pH increases), the thickener system can also produce a certain viscosity in the ground, and when the acid is consumed to a high pH, these systems will be diluted to improve the backflow ability after operation.
The purpose of the replacement fluid is to displace the acid in the wellbore and push the acid to the depth of the fracture to increase the penetration distance of the acid. When thickening acid is used, a large amount of acidified replacement fluid is very effective in increasing the length of the acid etched fracture, which is an extremely important step in the operation design. Higher injection speed is also advantageous.
closed acid fracturing
One process that has attracted interest in improving the ultimate conductivity to the wellbore is the Closed Fracture Acidification Process (CFA). This process was developed in the 1980s. It involves pumping a small amount of acid into a closed fracture below the reservoir rupture pressure to create as much open fracture as possible with the wellbore, in order to form an open fracture around the wellbore. This is similar to chasing a small amount of proppant to ensure communication between the wellbore and a conductive acid etched fracture.
In order to take full advantage of acid fracturing technology, the following points must be considered:
It is necessary to distinguish between acid fracturing and sand fracturing, their essence is different.
The advantages of acid fracturing can only be verified through field application and effect evaluation, rather than through current laboratory comparisons.
The development and application of new methods, including simplified process design, should be encouraged.
Acid fracturing will be widely used in highly studied mining areas (carbonate rock and sandstone).
Acid fracturing can also be considered in sandstone reservoirs.