Optimizing top hole drilling

Transcription

1 Optiizing top hole drilling This worksheet deterines the optiization of top hole drilling and wellbore cleaning based on the sedientary foration characteristics present and conditions applied. Optiized drilling rates and constraints then being identified to deliver the 'technical liit or loss free perforance' in conjunction with local operating knowledge, field application, experience and pressure while drilling data that ay exist. This worksheet additionally reviews effects of Annular: drilled cuttings concentration e.g. necessary to prevent pack-off, ud rings and hole cleaning difficulties pressure effects that ay result in foration (fracture) breakdown or (pressurized) well flow. hole size, section length, rates of penetration, foration density/characteristics, sweep volues desired. Cuttings Concentration & Pressures Cuttings concentration (Ca) and pressure effects while circulating and drilling in an annulus volue of a specified section length is dependent upon the following; Rate of penetration, (ROP), ft or /. Porosity (φh %) & density (Fd, lb/) of foration drilled. Flow rate puped (Q, or liter/in). Transport efficiency of drilled solids (Et, %). Drilling fluid density (ρ, lb/). Wellbore diaeter (D, in) Section length drilled (S1, ft or ) Note: Cuttings drag, shape, pipe rotation, swab/surge, hole angle, are not considered in the worksheet calculations presented. As these will have inial effect on large, vertical riserless wellbore sections drilled and forulae used. Suppleentary Notes: 75% of offshore sedientary forations are Claystones with a +/- icron particle size. Claystone will disperse in seawater and very high transport ratio's will exist i.e %. Note: PWD data fro recent riserless wells drilled have supported such facts. The other doinant foration is Silt/Sands with particle sizes fro i.e. several hundred ties > than clay particle. Sands are therefore very difficult to clean and transport ratios can be 30% or even lower. At the seabed the foration sedients have approxiately 70% porosity and pereability that decrease rapidly with burial & copaction on first,000-,000ft drilled depending on depositional settings environent etc. These first principle stateents are key fundaents to how fast clay foration can be drilled and cleaned (they siply disperse) and why sands are a different proposition due to ass, volue and cleaning required. In 100% soft clays; a range of cuttings concentration (Ca) fro > 6% - 8% by volue have been stated to result in Wellbore probles ranging fro inor ud rings, to ajor gubo or Wellbore pack-offs. Concentrations that need to be aintained when drilling and constrained as required. Cutting concentration effects in the annulus ay be liited by fracture gradient of these forations, and how fast the Wellbore can be drill or cleaned. If exceeded Wellbore overload, where fracture, instability and loss of the Wellbore, stuck pipe can occur if not anaged and controlled. Hence calculations are required?. This worksheet is valid for Vertical wellbore's up to 30degrees inclination. The calculations evaluate deterinants as input in the worksheet data input sheet that follows on page.

2 Worksheet data input bbl := Based on paraeters as input, concentration of cuttings (Ca) and pressure effects in the fluid annulus are deterined & evaluated. φ = Average section length porosity of foration (s) being drilled (%) Hs = Hole size(s) (inches) Fd = Foration density (lb/), (kg/ 3) Note: Typically 18ppg for claystone, ppg for sands ROP = rate of penetration, (ft/ or /) Q = drilling puping flow rate ( or liter/in) ρ = Fluid/Mud weight beng puped (lb/, kg/ 3 ) D = hole size drilled (inches,) C d = Previous casing internal diaeter (inches, ) d = drill string diaeter (inches, ) Wo = Hole enlargeent factor (diensionless) Single = Length of drillppe single (ft,) Stand = length of drillstring stand (ft,) Sl = Section length fro seabed (ft,) Cl = Conductor or previous casing length (ft,) φ := 5% Hs := 17.5in Fd := ROP := Q := ρ := D C d d 18 lb in 8.8 lb := W o := 1.1 Single Stand Sl Cl := 5 in Excess = Sweep excess volue to be puped. Excess := 1.5.5

3 Deterine annular cuttings volues. For average foration porosity, hole sizes and foration density input. Step #1, # & #3 deterine cuttings volue (Ah) and concentration (Cd) per distance drilled. Note; for non circulating conditions. Step # 1: Calculates the volue of cuttings drilled for hole size, (Hs inches), based on average porosity (φh, %) in ters of area (Ah) π ( 1 φ) Hs and volue per distance drilled (Ah, 3 Ah := Ah = /) Step # : Calculates the cuttings concentration (Cd, lb/f) now based on the Wellbore volue of cuttings drilled per foot (Ah) for a derived foration density (Fd, lb/), Cd := AhFd Cd = 553 lb Step # 3: Converts the cuttings concentration (Cd, lb/) into a ore identifiable drilling easureent i.e. a drill pipe single or a drilling stand for the various hole sizes (Hs, inches) as input. Cd = 3 ton Cd = 8 ton Transport efficiency & sweep volues required Step # ; Calculates the annular velocity (Av, ft/in) based on flow rate (Q), Wellbore diaeter (D), Wellbore enlargeent (Wo) and drill string diensions (d) as input. Av := DW o Q ( ) d Av = 53 ft in Notes: Drilled riserless sedients (cuttings?) are transported dependant upon fluid properties, flow (i.e. pup rate), drilling rates (ROP) and are influenced by foration characteristics e.g. porosity density, wellbore & drill string geoetries, fluid velocities & transport efficiencies. Fro Sifferans table when considering water, a thin or interediate drilling fluid. Fluid transport ratios are deterined and fro these cuttings concentration and pressure effects in the drilled annulus evaluated based on a range of penetration rates. Claytones, due to particle size disperse when drilled with any fluid** and therefore high transport ratios result i.e %. **This can be supported with PWD data. Step # 5; The inverse of annular ud velocity (1/Av) is then deterined fro Sifferans transport ratio's table and Av as deterined in step# * Drilled seawater will contain a ix of predoinantly dispersed clay that in essence has becoe a weighted fluid syste and thus a ore efficient transport ediu!. Step # 6; By plotting the inverse ud velocity (1/Av) and intersecting with the drilling fluid used. A transport ratio, (Et, %) is obtained. 1 Av = in Et := 50% ft

4 Sweep volue. Step #7; Sweep volue to aintain a fluid interface and clean the drilled sedients (cuttings?) fro the wellbore are deterined per single or drilling stand drilled and excess applied as follows. Sw vol := Ah W Et o Single Excess Sw vol = 3 bbl Sw vol = Sw vol := Ah W Et o Stand Excess 7 1 Sw vol = 97 bbl Sw vol = Cuttings concentration Step #8; Concentration of drilled cuttings (C a ) in the annular wellbore is now deterined fro the following equation for rates of penetration as input; Note: this deterines the effects of drilling ROP, circulating rate, drill string, hole diensions for pup rate and transport efficiency. Ca := ( ) ROP π ( 1 φ) D d EtQ Ca = % NB. Cuttings concentration effect is applicable solely for clays and shale's and is required to deterine the ROP liits to prevent ud rings, ud balls, gubo, packing off etc fro resulting. For Silts and sands this effect is not considered applicable. Here annular pressure effects due to increase of ass volue that ocurs when forations are penetrated presents the greater risks. Annular pressure (fracture) effects Step #9; Based on values deterined in Step #8. The effective equivalent ud weight ( ρ e lb/) in the Wellbore annulus can be deterined for rates of penetration input. ρ e := Fd( Ca) + ρ ( 1 Ca) ρ e = lb Discussion; When working tough calculations inputting and varying paraeters as appropriate. The key deterinants driving and influencing cuttings concentration and annular pressure 'fracture' effects can be identified. For clay sedients, the 'effective ud weight' will not likely exceed foration breakdown pressures due to the high transport ratios that exist. However for silts/sands breakdown is ore likley. In conclusion all factors ust be considered when optiizing riserless drilling to prevent operational (loss) fro occurring. e.g. Hole pack off, shallow water or gas flow, stuck pipe etc and best practices to result.

5 Section length considerations, Sl Step #10; Wellbore volue (D), section length (Sl) and previous casing volues are now equated. Open hole volue (Ohv), hole enlargeent (Wo, %), conductor volue (Cv) and total Wellbore volue (TWv) are calculated. Oh v := ( Sl Cl) Note. For larger Wellbore sizes drilled (>D), greater volues, 'cuttings concentrations' and 'annular pressures liits' will result. ( ) π D d W o Oh v = 858bbl π C d d Cv := Cl Cv = 33 bbl Tw v := Oh v + Cv Tw v = 1191bbl Step #11; Calculates approxiate cuttings bottos up tie to the seabed (Tds) based on Total Wellbore volue (Tw v ), flow rate (Q) and transport efficiency (Et). Tw v 1 Tds := Tds = 105 in Q Et Tds = 1.76 Note. When accounting section length, hole size, pup rate and transport efficiency. It can be concluded that 'hole size' and section length increases the tie required to clean the Wellbore. As the Wellbore cannot be physically cleaned faster than it can be drilled, there is a point where circulating 'bottos up' exceeds ore than 1 for the rates of penetration being drilled. This equivalent 'lag' or tie-based factor then needs to be accounted for. In effect, section length affects the axiu riser-less drilling rates that can be drilled that is a function of foration characteristics, hole volues, flow rate, transport efficiency, section length. Cuttings concentration and section length Step #1; Calculate cuttings concentration accounting for the Wellbore volue, section length and bottos up tie at various rates of penetration applied.. Ca := ( ) ROP π ( 1 φ) D d EtQ Tds Ca = Annular Pressure effects for section length Step #13; Based on cutting concentration (Ca). Calculates the revised effective weight due to cuttings concentration (ρe, lb/), for section length, bottos up tie for rates of penetration applied. ρ e := Fd( Ca) + ρ ( 1 Ca) ρ e = Suary. Riserless Drilling liits can be evaluated with reasonable accuracy (suppleented by down hole Pressure while drilling (PWD) tools where the key risk is the accuracy of known and unknown G&G and foration characteristics notably pressure and fracture gradient regies % lb

6 10 Cuttings (Conc %) vs. ROP: Section length & flowrate applied Cuttings concentration (%) Ca % cuttings concentration ROP Rate of drilling penetration (/) 11 Annular pressure effect for ROP & section length depth 10.8 Annular cuttings concentration effect ρe lb annular pressure ROP Drilling rate of penetration