Well Performance Analysis in a Multiwell Gas Condensate Reservoir— Arun Field, Indonesia Presented at SPE Advanced Technology Workshop Well Testing in Gas Condensate Reservoirs 1-2 April 2000, Calgary, Alberta, Canada T. Marhaendrajana, Texas A&M University N.J. Kaczorowski, Mobil E&P (U.S.) T.A. Blasingame, Texas A&M University

Summary—Well Test Analysis A representative summary of the analysis and interpretation of well test data taken from the Arun Gas Field (Sumatra, Indo-nesia) (Single-phase gas analysis is used). 2-zone radial composite reservoir model is effective for diagnosing the effects of con-densate banking at Arun Field. Application of a new solution for the analysis and interpretation of well test data that exhibit "well interference" effects.

Summary—Production Data Analysis Analysis and interpretation of production data using (single-phase) decline type curve analysis: Permeability-thickness product Skin factor Original and movable gas-in-place Comparison of results from well test and production data analyses vary—but these variations appear to be consistent.

N General Information—Arun Field (Indonesia) Field Description Located in Northern part of Sumatra, Indonesia Retrograde gas reservoir One of the largest gas fields in the world Arun Field has 111 wells: 79 producers 11 injectors 4 observation wells 17 wells have been abandoned Field Description Arun Field N

Major Phenomena Observed at Arun Field Liquid accumulation near wellbore (conden-sate banking) Need to know the radial extent of the condensate bank for the purpose of well stimulation. Well interference effects (well test analysis) Well interference effects tend to obscure the "standard" flow regimes—in particular, the radial flow response. This behavior influences our analysis and inter-pretation efforts, and we must develop an alter-native analysis approach for well test data affected by multiwell interference effects.

Well Test Analysis Strategy Condensate banking phenomenon: Used a 2-zone radial composite reservoir model—the inner zone represents the "con-densate bank," and the outer zone represents the "dry gas reservoir." (reported kh-values are for the "outer zone") Well interference effects: Developed a new method for the analysis of well test data from a well in multiwell reservoir where we treat the "well interference" effect as a "Regional Pressure Decline." (This pheno-mena is observed in approximately 35 cases)

Well Test Analysis: Examples Well C-I-18 (A-096)—Test Date: 28 Sep. 1992 Radial composite effects. Multiwell interference effects. Well C-IV-01 (A-060) [Test Date: 25 Feb. 1993]

WT Example 1: Log-log Summary Plot Raw data Corrected Improvement of pressure derivative. Infinite-acting Reservoir Model (Does not include non-Darcy flow) Pseudopressure Functions, psi Condensate banking region. Higher mobility region. Closed boundary at 160 ft? (includes non-Darcy flow). Effective shut-in pseudotime, hrs

WT Example 1: Horner (Semilog) Plot Well C-I-18 (A-096) [Test Date: 28 September 1992] Condensate banking region. Higher mobility region. Shut-in Pseudopressure, psia Raw data Corrected Horner pseudotime, hrs (tp = 1.56 hr)

WT Example 1: Muskat Plot (single well pavg plot) Well C-I-18 (A-096) [Test Date: 28 September 1992] pp,avg = 1148.6 psia Data deviate from the "Muskat line" —indicating interference effects from surrounding wells. Onset of boundary dominated flow (single well analogy). Shut-in pseudopressure, psia "Transient flow" dppws/dDta, psi/hr

WT Ex. 1: "Well Interference" Plot (radial flow only) Well C-I-18 (A-096) [Test Date: 28 September 1992] Intercept is used to calculate permeability. Slope is used in the pressure correction. (Dpp')Dtae, psi (Dpp')Dtae <0, indicating multiwell interference effects. Dta2/ Dtae

WT Example 2: Log-log Summary Plot Well C-IV-01 (A-060) [Test Date: 25 February 1993] Raw data Corrected Improvement of pressure derivative. Pseudopressure Functions, psi Condensate banking region. Closed boundary at 330 ft? (includes non-Darcy flow). Infinite-acting Reservoir Model (Does not include non-Darcy flow) Higher mobility region. Effective shut-in pseudotime, hrs

WT Example 2: Horner (Semilog) Plot Well C-IV-01 (A-060) [Test Date: 25 February 1993] Condensate banking region. Shut-in Pseudopressure, psia Higher mobility region. Raw data Corrected Horner pseudotime, hrs (tp = 2.61 hr)

WT Example 2: Muskat Plot (single well pavg plot) Well C-IV-01 (A-060) [Test Date: 25 February 1993] pp,bar = 1573.5 psia Onset of boundary dominated flow. Shut-in pseudopressure, psia "Transient flow" dppws/dDta, psi/hr

WT Ex. 2: "Well Interference" Plot (radial flow only) Well C-IV-01 (A-060) [Test Date: 25 February 1993] Intercept is used to calculate permeability. (Dpp')Dtae <0, indicating multiwell interference effects. (Dpp')Dtae, psi Slope is used in the pressure correction. Dta2/ Dtae

Correlation of Well Test Results—Arun Field Maps: kh (outer-zone (gas) permeability). skin factor. non-Darcy flow coefficient. Radius of condensate bank. Correlation of non-Darcy flow coefficient and the permeability-thickness product (kh).

kh Map kh distribution ap-pears reasonable. 3 major "bubbles" of kh noted, pro-bably erroneous. kh shown is for the "outer" zone (when the radial compo-site model is used). kh Map

Skin Factor Map Skin factor distri-bution appears very consistent. Areas of "high skin" indicate need for individual well stimulations. Skin factors are calculated based on the "inner" zone of the radial compo-site model (when rc-model is used).

D (Non-Darcy) Map This map indicates a uniform distribution. "high" and "low" regions appear to be focused near a single well. Relatively small data set (30 points). D (Non-Darcy) Map

Condensate Radius Map Good distribution of values—"high" spots probably indicate need for individual well stimulations. Relatively small data set (32 points).

D (Non-Darcy)—kh Crossplot D-kh crossplot indi-cates an "order of magnitude" correla-tion. Verifies that non-Darcy flow effects are systematic. Slope = 2

Production Data Analysis: Arun Field Well C-I-18 (A-096) Limited history (no EURMB analysis possible). Erratic performance. Reasonable match on decline type curve. Well C-IV-01 (A-060) Good history (well was down for almost two years in 1993-1995). Sparse p/z data for EURMB analysis. Early data match on decline type curve is questionable. Late performance data deviate from material balance trend on decline type curve, indicat-ing "well interference" effects.

WPA Example 1: Well History Plot

WPA Example 1: Decline Type Curve Plot Fetkovich-McCray Decline Type Curve (No Well Interference Effects)

WPA Example 2: Well History Plot

WPA Example 2: EURMB Plot

WPA Example 2: Decline Type Curve Plot Fetkovich-McCray Decline Type Curve (No Well Interference Effects)

WPA Example 2: Decline Type Curve Plot Fetkovich-McCray Decline Type Curve (Includes Well Interference Effects)

Correlation of Production Analysis Results—Arun Field Production data analyzed using decline type curve analysis—single-phase (gas) pp and ta used. Flow Properties: Volumetric Properties: Maps: kh skin factor Crossplots: khWT—khWPA sWT—sWPA Plots: G vs. time EURMB vs. time kh vs. time Crossplots: G—EURMB EURPI—EURMB

kh shown is com-puted using decline type curve analysis on early production (pp and ta used). kh distribution ap-pears reasonable (albeit lower than WT estimates). A few "bubbles" of kh noted, these are probably erroneous. kh Map

Skin Factor Map Skin factors com-puted using decline type curve analysis on early production. Skin factors from WPA are lower than WT estimates.

khWT—khWPA Crossplot khWT estimates are clearly higher than khWPA estimates. khWT estimates are "current," khWPA estimates are "ini-tial." Variation is system-atic—decline type curve analysis uses earliest production data for kh (and s) estimates.

sWT—sWPA Crossplot sWT estimates are "current," sWPA estimates are "ini-tial." Could argue that this plot shows the "evolu-tion" of the skin fac-tor. sWPA estimates should be higher, tied to kh estimation in decline type curve analysis.

Gas Reserves from Decline Type Curve Analysis

Estimated Ultimate Recovery from Material Balance

Flow Capacity (kh) from Decline Type Curve Analysis

G (Decline Type Curve)—EURMB Crossplot G-EURMB crossplot indicates excellent agreement of com-puted results.

EURPI—EURMB Crossplot EURPI—EURMB cross-plot shows excellent correlation of results. Verifies that these analyses are consis-tent.

Well Performance Analysis in a Multiwell Gas Condensate Reservoir— Arun Field, Indonesia Presented at SPE Advanced Technology Workshop Well Testing in Gas Condensate Reservoirs 30 September- 1 October 1999, Houston, TX. T. Marhaendrajana, Texas A&M University N.J. Kaczorowski, Mobil E&P (U.S.) T.A. Blasingame, Texas A&M University

Well Test Analysis: Examples (extra) Well C-IV-11 (A-084)—Test Date: 5 Jan. 1992 Multiwell interference effects. Well C-IV-11 (A-084)—Test Date: 4 May 1992 Radial composite effects.

Example 3: Log-log Summary Plot Well C-IV-11 (A-084) [Test Date: 5 January 1992] Raw data Corrected Improvement on pressure derivative. Pseudopressure Functions, psi Closed boundary at 150 ft? (includes non-Darcy flow). Infinite-acting Reservoir Model (Does not include non-Darcy flow) Effective shut-in pseudotime, hrs

Example 3: Horner (Semilog) Plot Well C-IV-11 (A-084) [Test Date: 5 January 1992] Shut-in Pseudopressure, psia Raw data Corrected Horner pseudotime, hrs (tp = 1.62 hr)

Example 3: Muskat Plot (single well pavg plot) Well C-IV-11 (A-084) [Test Date: 5 January 1992] pp,bar = 1920 psia Onset of boundary dominated flow. Shut-in pseudopressure, psia "Transient flow" dppws/dDta, psi/hr

Example 3: "Well Interference" Plot (radial flow only) Well C-IV-11 (A-084) [Test Date: 5 January 1992] Intercept is used to calculate permeability. Slope is used in the pressure correction. (Dpp')Dtae, psi Presence of multiwell interference effects is unclear Dta2/ Dtae

Example 4: Log-log Summary Plot Well C-IV-11 (A-084) [Test Date: 4 May 1992] Raw data Corrected Improvement on pressure derivative. Pseudopressure Functions, psi Condensate banking region. Infinite-acting Reservoir Model (Does not include non-Darcy flow) Closed boundary at 197 ft? (includes non-Darcy flow). Higher mobility region. Effective shut-in pseudotime, hrs

Example 4: Horner (Semilog) Plot Well C-IV-11 (A-084) [Test Date: 4 May 1992] Condensate banking region. Shut-in Pseudopressure, psia Higher mobility region. Raw data Corrected Horner pseudotime, hrs (tp = 1.63 hr)

Example 4: Muskat Plot (single well pavg plot) Well C-IV-11 (A-084) [Test Date: 4 May 1992] pp,bar = 1882.8 psia Onset of boundary dominated flow. Shut-in pseudopressure, psia "Transient flow" dppws/dDta, psi/hr

Example 4: "Well Interference" Plot (radial flow only) Well C-IV-11 (A-084) [Test Date: 4 May 1992] Intercept is used to calculate permeability. Slope is used in the pressure correction. (Dpp')Dtae, psi (Dpp')Dtae >0, no clear indication of multiwell interference effects. Dta2/ Dtae