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Application study of hydraulic jet fracturing to the fracturing of horizontal fracture in shallow reservoir
Abstract
The Chang-6 reservoir in Qilicun oilfield is a shallow reservoir with ultra-low permeability, low pressure and low yield. Conventional hydraulic fracturing can fracture 1 layer or 2 layers. The laminar reservoirs which contain several calcareous interbeds have low development degree because of bad vertical connectivity or no vertical connectivity. So the reservoirs have low yield and recovery factor. The multi-horizontal fracture fracturing of the laminar reservoir is effective way for enhancing their yield and recovery factor. The main factors of influencing the hydraulic jet fracturing effect of cluster wells in shallow reservoir are studied based on the hydraulic jet fracturing mechanism, and the technological measures for multi-horizontal fracture fracturing in shallow laminar reservoirs are proposed. The pilot test results show that, the hydraulic perforation-fracturing united technique can accurately determine the cracking location of horizontal fracture, which solves the uncertainty of the cracking location of horizontal fracture in the construction of wax-ball temporary plugging small-spacing multi-horizontal fracture fracturing technology. The pilot tests achieved good results.
With the development of Changing Oilfield exploration and exploitation, low porosity and permeability reservoir has become a major object of exploration and development. The reservoirs of almost all wells go through fracturing transformation before they have the capacity. The brittle rock sectional variation should be considered in reservoir fracturing, as brittleness coefficient indirectly reflects the degree for difficulty of the reservoir pressure to open from rock mechanics angle. Taking the Yanchang reservoir of Changing Oilfield in Jiyuan area for example, we get the rock brittleness coefficient and calculate the dynamic rock brittleness coefficient of calibration logging with the core of triaxial test data (Young's modulus and Poisson's ratio), by this we establish the continuous rock brittle coefficient of profile. Quantitative relationship between the rock brittleness coefficient B and minimum horizontal stress Sh is established, and quantitative prediction of fracture height and width is realized based on reservoir rock brittleness coefficient. Application of this method to the work area wells fracturing renovation project generated good results, which proves its worth of wide application.
It is determined that the hydraulic fracturing fractures of Qilicun Oilfield are in horizontal direction by the micro seismic detection analysis of four fractured wells, so the multi-fracture fracturing technology in one oil layer is suitable to Qilicun Oilfield. According to the statistical analysis of the yield of the fractured wells with one-fracture, two-fracture and three-fracture in one layer and the current fracturing construction technology, the parameters of the two-fracture fracturing technology in one layer, such as the fracture length, construction flow rate, sand amount, etc., are optimized by means of previous fracturing operation statistical data and simulation software. The main optimal fracturing parameters of Qilicun Oilfield are determined as following: fracture length is 35~42 m, construction flow rate is 1.8~2.2 m3/min, and sand amount is 64.10~92.30 m3. The yields of the two-fracture well with optimized fracturing parameters, one-fracture well of not optimized fracturing parameters, two-fracture well of not optimized fracturing parameters are compared, and it is shown that, two-fracture well of optimized fracturing parameters has the highest yield and stable yield, which proves the multi-fracture fracturing technology in one oil layer is suitable to Qilicun Oilfield again.
On the basis of erosion wear theory of materials and the abrasive water jet cutting process, the mechanisms of both casing cutting and formation rock cutting with hydraulic sand blasting perforation of oil wells are discussed, and the influencing parameters are investigated. The preliminary experiments of hydraulic sand blasting perforation both in laboratory and on field are conducted. The simulation experiment in laboratory shows that the casing can be easily cut with the hydraulic sand blasting perforation. The sandstone can be perforated, and a hole with 30 mm in diameter and 780 mm in depth can be made on the sandstone at jetting pressure of 23-24 MPa. The field tests show the promising potential enhancement of oil production with the technique. The improvement of oil production with hydraulic sand blasting perforation is mainly resulted from the removement of the near-well-bore pollution, looseness of compacted zone, increase of the formation permeability, which extends the passage-way of oil flow.
Experimental researches were introduced by selected samples from Chang 6 formation of Changqing Oilfield, and the process of hydrajet perforating and fracture initiation process was simulated by using field parameters, jetting tools and equipments. The tested samples were all cut by special tools. The experimental results show that the whole jetting hole is of tear-drop shape, and it includes four parts of casing perforation, entrance sputtering section, cement reduction and main section. When fracture initiated, a slim channel with gladiate shape is added in the end of the hole. The sample will fracture when jetting velocity is high enough in test condition, which indicates that the pressure in jetting hole increases obviously with the jetting velocity increase. The jetting flow erodes out a slim channel along crack propagate direction when fractures initiate, which makes the jetting hole length increase doubly, and is beneficial to enhance penetration efficiency. Individual channels are connected by crack when fractures initiate, which indicates that jetting hole can guide the fracture initiation and extension near the well bore in test condition.