Smart, horizontal, and multi-lateral wells provide an attractive option for maximizing reservoir contact in both oil and gas fields in Saudi Arabia. Formate drill-in fluid has been selected lately for drilling gas wells, because of its ability to preserve conventional polymers at high temperatures. The formate salts also can reduce the rate of hydrolytic and oxidative degradation of many viscosifiers and fluid loss agents at high temperatures.
Drilling fluid filter cake can reduce the flow efficiency of cased or openhole wells. Calcium carbonate particles are frequently used as weighting material to maintain the pressure that is required for well control and minimize the leak-off. Such solids become consolidated and trapped in the polymeric material and this makes the filter cake a strong permeability barrier.
The conventional method for cleaning the filter cake is by circulating solids-free formate brines at a high flow rate. This mechanical technique removes only the external drilling fluid damage. Chemical fluids like acids, oxidizers or chelating agents are usually used as an alternative method for dissolving both the external and internal filter cake. Most of these fluids cannot give a full coverage to the wellbore due to the formation heterogeneity.
A new class of esters can generate an acid downhole at a low rate and distribute it uniformly through the wellbore. Fluid compatibility and thermal stability tests were studied in detail. Return permeability experiments were conducted by using HPHT dynamic fluid loss cells. The ester solution was effective in cleaning the filter cake in comparison to the formate brines completion fluid and other organic acids results. Also, circulating formate brine to restore the wellbore permeability or remove the internal filter cake damage was found to be insufficient. Surface tension measurements indicated that the generated alcohol and lactic acid from the ester solution had lowered the surface tension more than the formate brines.
Introduction
The objective of multilateral-well technology is to improve the well productivity by maximizing reservoir contact, resulting in field development with a fewer number of wells. Long horizontal wells (up to eight km) are drilled in Saudi Arabia fields, but the greatest opportunity, and technological challenge, lies with MRC wells. A Maximum Reservoir Contact (MRC) well definition is a long horizontal well with more than five km of total contact with the reservoir rock.1
Drilling fluids are either water-based or oil-based. These fluids are made up of a base fluid, water or diesel (HC), weighting agents and other additives that aid in removing the cuttings from the well and keep the mud in a fluid state. Traditionally, the drilling industry has used water-based muds (WBMs) because they are inexpensive and have minimal impact on the surrounding environment. The density of the WBMs can be increased by adding calcium carbonate particles. High levels of suspended solids, however, in drilling fluids can cause high frictional losses during fluid circulation. These loses create high ECDs and limit pump rates. Low pump rates can result in inefficient hole cleaning.2
In difficult drilling situations, such as horizontal, extended reach, deep wells, and reactive shales, drillers rely usually on oil-based muds (OBMs). However, despite their high-performance properties, OBMs are costly to dispose-off and contain toxic materials, such as mineral oil.
The main concern with WBM is the thermal degradation of chemical additives that often occurs while drilling high temperature wells. Such degradation can lead to strong variations in rheological and filtration characteristics and loss of fluid properties.3 Moreover, even if no degradation of components occurs, the viscosity of hydro-soluble polymer solutions commonly used in drilling fluid formulations strongly decreases at high temperatures. Improperly formulated and maintained drilling fluid systems can cause significant near-wellbore formation damage and create potential for the plugging of screens and slotted liners.4
