A calculation model based on effective medium theory has been developed for predicting with a differential method. All types of pores are simultaneously introduced to the composite during the differential iteration process according to the ratio of their volume fractions. Based on this model, the effects of pore structures on predicted pore-pressure in carbonates were analyzed. Calculation results indicate that cracks with low pore aspect ratios lead to pore-pressure overestimation which results in lost circulation and reservoir damage. However, moldic pores and vugs with high pore aspect ratios lead to pore-pressure underestimation which results in well kick and even blowout. The pore-pressure deviation due to cracks and moldic pores increases with an increase in porosity. For carbonates with complex pore structures, adopting conventional pore-pressure prediction methods and casing program designs will expose the well drilling engineering to high uncertainties. Velocity prediction models considering prediction in carbonates.
In this article, we prepare the physical model with the high density epoxy resin and different sizes of coin shaped silicone pieces which can simulate the directional arrangement of fracture medium. On the basis of this, we analyze the influences of fracture angle on the ultrasonic compressional wave velocity using the pulse transmission method and compare the experimental results with the Hudson effective medium model. The experimental results show that, velocity decreases linearly with the increase of fracture angle within the interval of [0°~90°]. In a similar aspect ratio and same angle case, the increase of fracture porosity will result in the decrease of the velocity which is weakened by the increase of fracture porosity. At high fracture porosity, fracture angle becomes the important influence factor on the velocity changing. The calculation results of the Hudson model have a small deviation which is less than 5% compared with the experimental results but show a difference in different angle interval. The combination of the two methods will provide a certain basis for the indoor experiment and the measurement of the velocity in field.
Crack universally existing underground is an important kind of pores. In order to study the elastic wave propagation in fractured medium through experiment, a new method to make artificial core with certain cracks using oil well cement and camphor sheet or thin steel sheet is put forward. Geometric parameters of the crack, such as shape, size and aspect ratio, are approximately equal to that of camphor sheet or thin steel sheet. Using the thin steel sheet to make crack can be more easy and accurate to control the crack angle than using camphor sheet. The crack opening scales at millimeters. The aspect ratio of cracks formed by camphor sheet ranges from 1.4 to 8, and aspect ratio of cracks formed by thin steel sheet ranges from 2.5 to 70. This method is proved simple and feasible by experiment practice, which can provide artificial cores with certain crack for acoustic wave propagation study.
Changbei is a marginal tight-gas field in Yulin, west China. Because of high drilling costs associated with the Changbei gas field, operators focus on improving drilling performance by means of reducing drilling and tripping times. One effective way to maximize drilling efficiency is proper bit selection. This paper presents an improved bit design and successful field results from two representative sections: CB 17–2 and CB18–2, both 12.25-in. sections. Both applications were characterized by a 2500-m long directional section, usually drilled through varying formations ranging from soft abrasive to very hard, with compressive strength ranging from 7 to 20 kpsi. Three main challenges were encountered during this operation. First, when drilling the upper portion of the sections, a series of transitional hard/soft formations, such as abrasive sandstone/siltstone and claystone, were encountered, which generated extensive vibrations and decreased the rate of penetration (ROP). Secondly, the long soft claystone intervals and frequent thin but very abrasive sandstone stringers in the lower part of the section not only increased bit balling potential, but also created premature cutter wear and loss of aggressiveness. The third challenge using a polycrystalline diamond compact (PDC) bit in this area related to drilling the whole section in fewer trips and maximizing the ROP to reduce drilling costs. To overcome these challenges, extensive geological research was performed within the 12.25-in. section of the Changbei gas field. Then, an improved bit design proposal was implemented, which consisted of three aspects:apply new cutter technology to the bits,optimize the bit design based on directional and formation challenges, anddevelop optimum drilling parameters. Based on field results, this approach set both ROP and interval records compared to offsets and helped the operator save 3.26 drilling days_one trip and one bottomhole assembly (BHA) in Section CB17–2. Drilling efficiency for both sections was enhanced by more than 20% compared to offset wells. Furthermore, for both sections, the bit used exhibited improved dull conditions compared to offsets.
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