Wellbore Fill-Up

Key Takeaways

• Wellbore fill-up calculation methodology requires accurate knowledge of the wellbore geometry at each depth interval, including the casing sizes, liner tops, open hole diameters (from caliper log data), and the dimensions of any tubulars in the hole, to compute the capacity (volume per unit length) of each section and integrate over depth to get the total fill-up volume: the capacity of a cylindrical annular space is computed as C = pi/4 x (OD_outer^2 - OD_inner^2) x L, expressed in barrels per foot (bbl/ft) using the oilfield conversion factor 1 bbl = 9.702 cubic feet (or equivalently, capacity in bbl/ft = (OD_outer^2 - OD_inner^2) / 1029.4, with diameters in inches); a typical well with 9-5/8 inch casing (8.835 inch ID) and 5 inch drill pipe (4.276 inch ID) has a drill pipe annulus capacity of (8.835^2 - 5.0^2) / 1029.4 = 0.0514 bbl/ft in the cased section, or approximately 5.14 barrels per 100 feet; the fill-up volume for a specific section of the wellbore is the annular capacity times the depth interval length, summed over all intervals of different geometry; tables of standard pipe and casing capacities are published in API bulletins and drilling engineering handbooks and are carried by every drill crew for rapid field calculations, but modern integrated well control software computes fill-up volumes automatically from the wellbore survey and completion data input.
• Kick control and wellbore fill-up are directly linked because the circulation of a gas kick out of the wellbore using the driller's method or wait-and-weight method requires tracking the wellbore fill-up volume to determine when kill weight mud has reached specific points in the wellbore: in the driller's method (circulating the kick out with original mud weight, then circulating kill weight mud down to kill the well), the fill-up volume of the drill string (from surface to the bit) tells the driller how many pump strokes are required before the original-weight mud begins returning at the bit and gas migration through the kill mud begins in the annulus; in the wait-and-weight method (mixing kill weight mud at surface and then circulating it to the bit before the kick reaches the surface), the fill-up volume of the drill string is the critical volume that, when pumped, delivers kill weight mud to the bit and begins the pressure-controlled displacement of the gas-cut mud in the annulus; monitoring the surface casing pressure against the expected pressure profile as kill mud fills the annulus (the casing pressure should track the predetermined kill sheet graph that accounts for the changing fluid weights in the wellbore) confirms that the well control operation is proceeding according to plan, with deviations from the planned pressure trace indicating either inaccurate fill-up volume estimates or unexpected wellbore conditions requiring adjustment of the well control procedure.
• Cement volume calculations for primary cementing use the wellbore fill-up concept to determine how much cement slurry must be mixed and pumped to achieve the planned cement top between the casing and the formation: the cement volume calculation begins with the caliper log data that defines the actual hole diameter at each depth in the open hole section (which may differ significantly from the bit size due to washouts or under-gauge sections), computes the annular volume between the casing OD and the actual borehole at each depth interval, sums the annular volumes over the planned cement interval to get the theoretical cement volume, and adds a cement excess factor (typically 25-50% of the calculated volume for competent formations, higher for washed-out intervals) to account for casing eccentering and formation irregularities that increase the effective annular volume beyond the caliper measurement; the fill-up volume of the casing (the volume of the casing bore from the cement shoe at the bottom to the planned cement plug landing collar) determines how much displacement fluid (drilling mud) must be pumped after the cement to push the casing fill-up of cement out of the casing and into the annulus; pumping too little displacement fills the casing bore with cement that hardens and must be drilled out, while pumping too much displacement pushes the entire cement column into the annulus and leaves the casing bore with excess displacement fluid that will contaminate the cement if it bypasses the float collar.
• Production wellbore fill-up after a workover or temporary abandonment is required when a well has been shut in for an extended period and the wellbore fluid has either leaked off into the formation through perforations, evaporated from the casing (in shallow or dry wellbore conditions), or been intentionally removed for wellbore integrity testing, and production must be restarted from a wellbore that is partially or fully empty: pumping a wellbore to fill up from empty is a sensitive operation because the wellbore may have formation gas in the annulus (from slow gas migration from perforations during the shut-in period) and filling the empty wellbore with fluid creates increasing hydrostatic pressure that may exceed the fracture pressure of weak zones in the wellbore if the fill-up rate is too high; slow fill-up procedures (limited pump rate with pressure monitoring to prevent exceeding the formation fracture gradient) are used in wells where the wellbore integrity is uncertain after extended shut-in; the fill-up volume required to restore the wellbore to its pre-shut-in fluid level provides a quantitative measure of how much formation fluid (gas or liquid) has migrated into the wellbore during the shut-in period, which is useful information for the production engineer assessing the inflow performance of the well and the integrity of the mechanical isolation between zones.
• Workover fluid fill-up calculations ensure that the wellbore is properly loaded with kill fluid before performing open-hole or perforated interval workover operations that require the wellbore to be overbalanced (wellbore pressure greater than formation pressure) to prevent formation fluid influx: a workover that pulls the production tubing from a producing well before the well has been properly killed by replacing the production fluid with a heavier kill fluid (typically a clean brine weighted to provide the required overbalance at the perforated interval) may result in the wellbore becoming underbalanced as the lighter production fluid is displaced and the wellbore pressure drops below formation pressure; the kill fluid fill-up volume is calculated as the volume of wellbore from surface to the perforated interval less the volume occupied by any tubing and tools that remain in the wellbore, weighted by the density required to achieve the target overbalance; kill fluid fill-up calculations are a standard deliverable of the workover program that is reviewed and approved by the well control authority before any open-wellbore workover operations begin, ensuring that the kill fluid volume, density, and pump rate specifications are adequate to maintain wellbore integrity throughout the workover operation.