# Please help: In an assignment A(I) = B, the number of elements in B and I must be the same.

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Guilherme Torres on 20 Apr 2016
Commented: Kuifeng on 21 Apr 2016
I am getting this error: (Please tell me exactly what to change? I am desperate!!) In an assignment A(I) = B, the number of elements in B and I must be the same.
Error in Untitled (line 54)
P(j) = P(j-1)+ delta_p_old/2 * depths;
depths = zeros(1,41);
pressures = zeros(1,41);
% define constant variables
q_o = 500; % bbl/d
T_i = 130; % degrees F
Tubing_ID = 2/12; % feet
Depth = 8000; % feet
API = 28; % dimensionless
mu_l = 1.03; % cp
Y_g = 0.71;
mu_g = 0.0131; % cp
sigma_og = 30; % dynes/cm
pipe_roughness = 0.0006; %dimensionless
P_sc = 14.7; % psia
T_sc = 60 + 460; % degrees F;
delta_z = 0:200:8000; % change in depth iteration of 200 feet to 8000
limit = 42; % iteration limit
iter = 0; % starting iteration
P = zeros(1,41);
%P = zeros(1,41);
P(1) = 100;
depth(1) = 0;
%change_P_new(1) = 200;
%delta_p_new = 200;
error = 2378645;
% P(i) = 100; %psi
for i = 100:100:800 % for loop
q_g = q_o*i;
for j = 2:41
while error > 0.001
if iter ==0
delta_p_old = 200;
end
if iter > 0
delta_p_old = delta_p_new;
%change_P_new = delta_p_new
%change_P_new(j)= delta_p_old/2;
end
P(j) = P(j-1)+ delta_p_old/2 * depths;
%P(j) = P(j-1)+ delta_p_new(0:delta_p_old / 2:8000);
% if iter == 2
% P(i) = P(i) + delta_p_new;
%
%
% end
%for oil
Y_oil = (141.5/(131.5 + API));
rho_l = Y_oil * 62.4;
%for gas and Z factor
P_pc = 709.6 - (58.7 * Y_g);
T_pc = 170.5 + (307.3 * Y_g);
P_pr = P(j)/P_pc;
T_pr = T_i/T_pc;
A = 1.39*(T_pr - 0.92)^0.5 - 0.36*T_pr - 0.101;
B = P_pr*(0.62 - 0.23*T_pr) + P_pr^2 * (0.066/(T_pr-0.86) - 0.037) + (0.32*P_pr^6)/(exp(20.723*(T_pr - 1)));
C = 0.132 - 0.32*log10(T_pr);
D = exp(0.715 - 1.128*T_pr + 0.42*T_pr^2);
Z_fact = A + (1-A) * exp(-B) + C*P_pr^D;
rho_g = (P(j) * Y_g * 28.97)/(Z_fact * 10.73 * T_i);
u_sl = (5.615*q_o)/(86400*0.25*pi*Tubing_ID^2);
u_sg = (1/(86400*0.25*pi*Tubing_ID^2))*q_g*Z_fact*(T_i/T_sc)*(P_sc/P(j));
N_vl = 1.938*u_sl*(rho_l/sigma_og)^0.25;
N_vg = 1.938*u_sg*(rho_l/sigma_og)^0.25;
N_D = 120.872*Tubing_ID*(rho_l/sigma_og)^0.25;
N_L = 0.15726*mu_l*(1/(rho_l*sigma_og^3))^0.25;
CN_L = (0.0019+0.0322*N_L - 0.6642*N_L^2 + 4.9951*N_L^3)/(1-10.0147*N_L + 33.8696*N_L^2 + 277.2817*N_L^3);
H = (N_vl*P(j)^0.1 * CN_L)./((N_vg.^0.575)*P_sc^0.1 * N_D);
yloverpsi = ((0.0047+1123.32*H+729489.64*H.^2)/(1+1097.1566*H+722153.97*H.^2)).^0.5;
B = (N_vg*N_L^0.38)/(N_D^2.14);
psi = (1.0886-69.9473*B.^2 - 12896.683*B.^3)/(1-53.4401*B+1517.9369*B.^2-8419.8115*B.^3);
Liq_Hold_Up = (yloverpsi)*(psi);
rho_bar = Liq_Hold_Up*rho_l+(1-Liq_Hold_Up)*rho_g;
m_dot = (0.25*pi*Tubing_ID^2)*(u_sl*rho_l+u_sg*rho_g)*86400;
N_Re = ((2.2*10^-2)*m_dot)/(Tubing_ID*(mu_l^Liq_Hold_Up)*(mu_g^(1-Liq_Hold_Up)));
one_over_sqrtf = -4.*log((pipe_roughness./3.7065)-(5.0452./N_Re).*log((pipe_roughness.^1.1098./2.8275)+(7.149./N_Re).^0.8981));
dpoverdz = (1/144).*(rho_bar + (m_dot.^2 .* (1./(one_over_sqrtf.^2))./(rho_bar.*7.413*10.^10.*Tubing_ID.^5)));
delta_p_new = dpoverdz*delta_z;
error = abs(delta_p_new - delta_p_old);
iter= iter+1;
depths(j) = j * 200;
%pressures(
end
delta_z = delta_z - 200;
if (delta_z < -8000)
break
end
end
plot(P, depths)
end

Kuifeng on 20 Apr 2016
depths = zeros(1,41); %it has 41 elements and P(j) is one element.
% maybe you wanted to use
depths(j)?

Show 1 older comment
Kuifeng on 20 Apr 2016
%what are the variables shown in workspace in the line 82,
Z_fact = A + (1-A) * exp(-B) + C*P_pr^D;
Guilherme Torres on 20 Apr 2016
Those variables are coefficients to an production oil well. They are defined above on lines 77,78, 79, 80, 81. The complex number is created on line 81 (Z_fact). This needs be be consistent with the rest of my data types, which I have no idea how to do. Help?
Kuifeng on 21 Apr 2016
how about crop a picture of these variables in the workspace?