Stimulation of carbonate formations by acid dissolution of the rock has been an efficient and successful method of bimproving production in oil and gas wells. Hydrochloric acid is the normal fluid of choice. However, in high temperature applications corrosion issues limit usage, especially in chrome completions. Acetic acid has been used with some success and with adequate corrosion protection. But due to its low reactivity at higher temperatures, the efficiency with which a gallon of acid dissolves the formation is perceived as low. This perception comes from reaction efficiency of acetic acid reported in the literature ranging in values from 90% at 25 °C to 40% at 121 °C for 2 to 15 wt%, respectively. Acetic acid reaction on calcium carbonate is controlled by its small dissociation constant, 1.754E-05 at 25 °C (77 °F) and therefore is labeled a weak acid.
An interactive computer based drilling fluid hydraulic program was developed which takes into account the rheological models of Bingham, Power law and Herschel- Bulkley. The software developed in visual basic adds a new dimension to drilling hydraulics by including pipe roughness correction, eccentricity and the use of the Fann viscometer 6- dial readings in the hydraulic model. Field data of a well drilled with a water-based and oil-based mud in Oklahoma was used to validate the software. The measured frictional pressure losses for two (2) hole sections were matched with those obtained using the three models. It was observed that a single rheological model is not adequate as rheological behavior changes. That is, a drilling fluid rheology might match the power law model behavior at the surface and acts like a Hershel-Bulkley fluid at total depth. From the results, the Herschel-Bulkley model including pipe roughness correction gave an excellent prediction for pressure loss inside the drill string (96% accuracy) and the power law model gave good results (93% accuracy) for the annular pressure loss calculated.
Accurate frictional pressure loss calculation is important during drilling operation. It is useful in controlling subsurface pressures, evaluating pressure increases in the wellbore during mud circulation, removing cuttings from the well, minimizing hole erosion during circulation, controlling surge and swab pressures, and sizing surface equipment such as pumps etc. The search for crude oil and natural gas reserves in formation with extreme pressure variation necessitates the accurate prediction and control of the rheology of drilling fluids and hydraulics. Thus, an accurate estimation of frictional pressure losses is advantageous from the economic point. Optimizing friction pressure loss calculation through the entire hydraulic system allows an appropriate evaluation of the wellbore hydraulics, which is essential to reducing problems such as poor hole cleaning, stuck pipe etc.
Wellbore hydraulics optimization involves an adequate choice of fluid rheology, flow parameters, and wellbore geometry. In many cases, fracturing and drilling fluid flow behavior deviates from the simple Newtonian behavior to non- Newtonian. As a result, friction pressure loss predictive equations become complex and less accurate due to many simplifying assumptions. It has been established that the particular rheological model and the flow parameters used in the development of a given empirical correlation or theoretical expression to characterize the (typically pseudoplastic) non- Newtonian behavior of drilling fluid or cement slurry are pivotal to drilling hydraulics calculation.
A considerable number of empirical and th