Geochemical Characterization of Alteration of Saturate Fractions by Water Flooding

Key words: 
• water flooding /
• saturate fraction /
• reservoir geochemistry /
• secondary alteration

Abstract: Based on the GC and GC-MS analysis, modification of saturate fractions in waterflooded oils from Qu9 reservoir was dynamically investigated. The results indicate that with the advance of waterflooding, saturates characterize obvious alterations during 9-months dynamic surveillance. Saturate concentration, C₁₅-RD/C₁₅-D, C₁₅8β(H)-D/C₁₆8β(H)-HD, TTs/17α-H and RS/17α-H ratios, are progressively increasing in values. Aromatic concentration∑nC_21^-/∑nC_22⁺ and ∑nC_14^-/∑nC_15⁺reduce continually. Isoalkane related ratios of Pr/Ph, iC₁₈/Pr, iC₁₈/Ph, terpane related ratios of C₂₁/C₂₂TT, C₂₄/C₂₃TT, ∑C_19-26/∑C_28-29TTs, C₃₀^*/C₂₉Ts, C₃₅R/C₃₄R, Ol/C₃₀H, G/C₃₁R, C₃₁22S/(22S+22R), C₃₀M/C₃₀H, Ts/(Ts+Tm) and C₂₉Ts/(C₂₉Ts+C₂₉H), and sterane related ratios of C₂₇-DS/C₂₇-RS, C₂₉20S/(20S+20R), C₂₉ββ/(ββ+αα) and C₂₇-rearranged sterane 20S/(20S+20R), keep relatively unchanged. However, irregular variation of Pr/nC₁₇ and Ph/nC₁₈ shows an initial increase and subsequent decrease, or the opposite. Water washing leads to the decrease of aromatics and light/heavy alkane values, while the biodegradation results in the concentration of tricyclic terpanes and regular sterane compared to the hopanes. Although they have minor effect on the ratios respectively composed of two isoalkanes, terpanes and steranes with similar carbon numbers, they cause the increase of drimane parameters in combination. Mixed with the geo-chromatography and wettability transformation in water flooding, saturates compositions were further altered.