Downhole conditions

The rotary shoulder connections must be made up with such torque that the shoulders will not separate under downhole conditions. This is of critical importance because the shoulder is the only area of seal in a rotary shoulder connection. Threads are designed to provide a clearance between crest and root that acts as a channel for lubricant and also accommodates the small solid particles. 

The magnitude of the safety factor is very important and usually ranges from 1.4 to 2.8 depending upon downhole conditions, drill pipe quality and acceptable degree of risk. It is recommended that a value of safety factor be selected to produce a margin of overpull of at least about 70,000 lb. 

To an appreciable degree the coupon experiences the same downhole conditions as the drillstring does. Therefore, its condition represents the corrosive effects of the downhole environment. However, limitations of this test are that the coupon is only exposed to the inside of the drillstring and not subject to the Same stresses. The results obtained are only for certain depth of exposure, while the corrosion may vary appreciably up and down the hole. Finally, the results are not available until the tool is pulled out of the hole. 

A further difficulty that faces oil field modelers is the lack of information they have about downhole conditions and reservoir characteristics. Forward predictions about production are distressingly uncertain. It is therefore common to fit observed production data retrospectively—a procedure known as history matching—and to infer reservoir parameters, particularly permeabilities and relative permeabilities. 

In continuous treatment Nowcorr 800 is added to the drilling fluid or to the acid blends. The concentration depends on downhole conditions temperature, total pressure and... 

When components of CBS slurry dissolve in water, their aqueous ions may have a different ionic size as compared to that in the unreacted crystal, and hence, the volume of the dissolved species may be different from that of the unreacted components. Thus, the total volume after dissolution Vf may be different from the total volume prior to dissolution Vq-This volume change will occur against the downhole pressure, because the slurry components will be mixed during pumping and wiU dissolve when they are placed in downhole conditions. Thus, the change in enthalpy of an individual component is given by... 

Foamed fluid rheological properties are very responsive to changes in foam quality. Therefore, in order to achieve the desired treatment goals, accurate metering of the constituents of the foam is essential. The liquid C02 rate is measured using a low-pressure turbine metering device. To determine the volume of C02 required at the downhole conditions, a mass balance across the total system can be accomplished. 

Foams are fluids that depend on shear history. The texture of a foam will reach an equilibrium state at a particular shear rate. Finer textured, more dynamically stable foams are produced at high shear rates, higher pressure, and with higher quantities of surfactant (36). Reidenbach et al. (11) observed that at higher shear rates, finer more uniform bubbles were created. This information indicates that at downhole conditions during fracture stimulation when conditions of high pressure and shear are present, foams are finely textured with parallel-piped uniform bubbles. 

Because of the nature of foams, the rheological character of foam fluids is difficult to quantify. The viscosity of foams is primarily dependent upon foam quality and external-phase fluid viscosity. Stimulation fluids are subjected to pressure variations from surface to downhole conditions therefore, foam quality and viscosity will change accordingly. In order to overcome the changing conditions experienced during stimulations, the foam fluids are designed to reach a specified quality at downhole conditions. 

Prior to choosing the operating conditions for prospective corrosion experimentation, it is pertinent to consider the actual field downhole condition and then simulate those conditions in the laboratory. 

There are reports presenting the downhole conditions of various reservoirs both home and abroad. Some of them are highlighted below. 

Ron et al. (2006) reported that downhole conditions may vary from one region to another. They studied that drilling pressures and temperatures can be from a range of low temperatures and pressures of 15 psi/75°F to HTHP conditions of about 40,000 psi/600°F especially in the hotter regions and deep reservoirs of the Gulf of Mexico Continental Shelf, northern India, Saudi Arabia, Brunei, Indonesia, Thailand, and northern Malaysia. 

Downhole specimens are rarely used for monitoring because of the costs and complications involved. Indirect monitoring of downhole corrosion conditions has been done with specimens or electrical resistance probes installed at the wellhead. These have been used for monitoring inhibition treatments, always with the reservation that such exposure does not accurately simulate downhole conditions. 

To predict the transport properties and performance of fiber sweeps under downhole conditions, the rheological properties of the base fluid and suspension must be understood. The proposed formulations for such fiber sweeps will be most effective when the rheology has been accurately modeled and fine-tuned for specific wellbore conditions. To begin to grasp how the fluid behaves, the relationship between shear stress and shear rate must be known. This is denoted as the shear viscosity profile, which is an aspect of the rheology of a fluid that is thought to control the hydrodynamics of flow. The most common shear viscosity models used in the oil and gas industry to characterize non-Newtonian drilling fluids include ... 

In order to more closely approximate the behavior of the fiber fluid under downhole conditions, the ambient temperature experiments were repeated at a somewhat higher... 

A mathematical model was developed to describe the effect of viscosity on the stability of fiber-containing drilling fluid sweeps. In order for the fiber to perform as efficiently and effectively as possible as a hole-cleaning aid, it is important that the fiber be homogeneously distributed throughout the base fluid, i.e. minimal separation of the fiber occurs under downhole conditions. Thus, a theoretical study was conducted to determine the desirable base fluid properties to formulate sweep fluids that are stable under downhole conditions. 

Selecting the type of polymer used for drilling sweep applications is critical in designing fluids that have good stability under downhole conditions. XG polymer appears to be a better choice than PAG or PHP A. 

Demirdal, B., Miska, S., Takach, N.E. Cunha, J.C. 2007. Drilling Fluids Rheological and Volumetric Characterization Under Downhole Conditions. Paper SPE 108111 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference, Buenos Aires, Argentina, 15-18 April. 

Yamamoto, S. Matsuoka, T. 1993. A Method for Dynamic Simulation of Rigid and Flexible Fibers in a Flow Field. /. Chem. Phys. 98 (1) 644-650 Yu, M, Takach, N.E., Nakamura, D.R. Shariff, MM. 2007. An Experimental Study of Hole Cleaning Under Simulated Downhole Conditions. Paper SPE 109840 presented at the SPE Annual Technical Conference and Exhibition, Anaheim, California, 11-14 November. 

Control of thermochemical processes under downhole and reservoir conditions is key to both work safety and optimization of enhanced oil recovery methods. In 2010, Emmanuel Institute of Biochemical Physics presented to Rostechnadzor a mobile laboratory that controls reaction under downhole conditions and ensures safe injection of large amoimts of nitrates into the reservoir. Rostechnadzor approved experimental injection of an unrestricted amount of nitrates into boreholes under the requirement of two levels of safety control [5]. 

The shift in equilibriirm is so dramatic that path (2a) remains predominant even under downhole conditions where nitrate decomposition products are in a dense fluid state. For example, the heat of reaction at a downhole temperature between 500 and 700 K should not change by more than 10% relative to standard conditions. Estimates for the energy budget imder downhole conditions may be obtained without taking into accoimt variations in heat of reaction and product composition with temperature and pressure. 

Packer systems for acid placement in horizontal wells do exist, however, and can be effective. The limitation that does exist with such systems is the number of placement settings or injection stages that can be applied. Typically, the rubber packer sealing elements can withstand 10-15 settings in a particular well condition before the risk of packer failure becomes high. The number of settings depends on the packer and downhole conditions. 

Although these tests somewhat simulate downhole conditions, there is such a variety of actual environmental conditions and types of drilling fluids that field testing is still the only sure method to check the performance of various elastomers. 

The overall completion process can also cause design difficulties since the mixed bag of fluid types above may occur at different times in the life of a producing well. The chemistry of production enhancement techniques is extremely complex and varies wildly with the downhole conditions. To add to this, extremes of pressure... 

Shokoya, 0. S., Al-Marhoun, M. A, and Ashiru, 0. A (1996). The Rheology and corrosivity of water-base drilling fluid under simulated downhole conditions. Mater. Corrosion/Werkstoffe und Korrosion, 47,307-316. 

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